No way am I going to touch the gas/flue side of things. I was toying with the idea of fitting a new PCB as that's relatively straightforward - just unplug the old one and plug a new one in but as I've never had it serviced, I'll get that done first by a specialist and take it from there. Thanks for all your advice, I'll let you all know how it goes. Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

Steve
Give British Gas a call, they have a fixed price breakdown charge.
Once they have attended they are committed to completing the repair. As with all things an intermittent fault can be a to diagnose.

On another note the governing body for the gas industry is no longer Corgi, it is now Gas Safe.

 Now at the point when I learn something new something old is lost out the other side !
Now retired so it doesn't matter anymore.
Freelander now gone.

call bg and they will be recommending a new boiler/power flush and anything else they can sell you.go for a reliable independent.not treading on your toes gasman

Sorry but that's just plain rubbish.
If they did that they would soon be out of business.  Now at the point when I learn something new something old is lost out the other side !
Now retired so it doesn't matter anymore.
Freelander now gone.

Personally from my experience on diynot.com British Gas do indeed try and fleece people with scare stories. My parents for example still call them the Gas Board and weren't aware there were independent companies who could fix boilers. That is how they make their money.

I also wasn't suggesting the OP tinkered with anything other than taking the case off to see if the fault went away. I would then still be putting the case back on and calling someone out but with the ability to better explain the symptoms. Much like we do on our cars  Learned to drive in 2011.
Bought a Micra - great little car
"Upgraded" to Kia Sportage - hated it
Proper upgrade to FL2 TD4 GS 60 reg Facelift - tailgate paint issues, fuel gauge rewire, transfer box failure at 36k miles, clutch at 60k
Reconditioned diff at 70k ish
Sold at 84k and now on a DS. Terribly unreliable.
Sold DS for a 2021 Mitsubishi Outlander PHEV. Fingers crossed....
Sold Outlander for a 2014 SD4 SE Tech because I’m mental

BOOM.  Now at the point when I learn something new something old is lost out the other side !
Now retired so it doesn't matter anymore.
Freelander now gone.

Typical, damn thing has been working faultlessly since I first asked for advice but I think I'm still going to get it serviced.

I must admit to being reluctant to get BG in though, based on my only previous experience with them. A few years ago my aged parents moved into a bungalow near me and as I was sorting everything out and not knowing the history of the heating system, I signed them up for an annual maintenance plan with BG. After they had been 'insured' with them for more than three months and the heating had been working fine, BG finally came round to inspect/service the boiler. The fitter claimed numerous faults with it and some really (I thought) petty stuff including fitting a grille over the flue and installing some kind of electrical heater element in the condensate drain-off to stop it from freezing (bear in mind this was just after we had had one of the coldest winters on record and it hadn't frozen up then!) and they wanted something like £750 for the total works before they would continue with the service and guarantee. I told them to sling their hook and got my parents payments back. Had it serviced by a local company within a few days of that and a few bits done on it for less than £150 and it's been fine ever since. That reminds me, maybe they should have another service too!!

So, if I get a service by any Gas Safe engineer (independent or BG) what exactly will/should they do? Like some garages or main dealers, do they all carry out their 'own' service or is there a standard Gas Safe list/check of things that should/must be done? For example, as mentioned earlier by Archie, will I get the burner and pilot removed and cleaned? The flue and fan checked? And if so, will that be done as a matter of course or will I have to ask specifically for that to be done?

Finally, what's the going price for a boiler service? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

I'll leave you with this.
As BG are the largest in the domestic central heating market, their engineers are constantly audited by both internal and external organizations. They are also spot checked by Gas Safe. They also have to stick religiously to manufacturers instructions, gas regs and building regs. That is why you were recommend the flue terminal guard and condensate trace heater, this is part of current regulations.
Small independents can chose to ignore some regulations safe in the knowledge that they will not be checked.
It's called 'Gas Safe' for a reason.

I have seen the result of corner cutting.

I am retired therefore have no vested interest in your decision.
Flip a coin and see what happens.  Now at the point when I learn something new something old is lost out the other side !
Now retired so it doesn't matter anymore.
Freelander now gone.

If the boiler is 20 years old it really is time to get it replaced. Anyone you get to service it will say that, and BG definitely so. They are also famous for saying they can't get the parts, when said parts are readily available on the internet. I'm sure they are often OK but I've read a lot of stories about them.
It's an old boiler so a service is generally just a vacuum and clean out the jets, as stated. It sounds to me (I'm not a gas engineer) as though other bits are starting to play up as well, and a new boiler, which will be much more efficient, is the way to go.
A friend of mine has just decided to replace their dead boiler (I've just spent a week in their unheated house relearning the art of fire lighting). Interestingly the quote from BG was about £100 less than the local man, but they decided to go with him as they preferred the make of boiler he will install. It's actually their pump that has gone, and they do have a contract with BG, but although BG would have replaced the pump they pointed out that the system was full of crud and it would go again shortly, plus the boiler is on its last legs (ie, they wanted to sell them a new one).
Three things if you do get a new boiler -
1- if you or your parents fulfill certain financial criteria, grants are available;
2- if replacing the boiler the system should be power flushed;
3- modern boilers are much more complex and really should be serviced annually. My local man spends around an hour on the job and charges £60.00, which I think is pretty good value, but then I do live out in the backwoods. When I lived in London, I found the best way to find an engineer was to get a recommendation from my insurance company. They found me a fitter who would only come out after hours, but he was still cheaper than the others. I've also used the boiler manufacturers' service - they have a tie up with local plumbing firms. Ex AA Series III LWB Safari - Gone
300TDi Disco (bought new - terrible car) sent back after 18 months
Freelander 1 Estate - leased, given back at end of lease
200TDi Disco (bought from a mate with 100,000 on the clock) - Gone
Disco 2 TD5 - sold and exported to France
FR2 TD4 GS - Gone
FR2 SD4 HSE - Now changed for a DS
New model ex-demo Evoque S 180 in white
Unable to order a new DS, so gave up. Now have a Volvo S90 Recharge.

Update:

I'VE FIXED IT! (I think).

Tried several heating specialists to get a service done first including BG and not one of them would come out for a service until mid to late January but luckily this morning it would not fire up at all. My boiler is in the loft so I went up and got the missus to keep turning the room stat on and off. Each time the stat was turned up, the pump started, the fan started, it fired up, run for ten seconds or so, flame went out and it tried to ignite again three or four times and then nothing but the pump and fan would run continuously. A bit of Googling when I got to work and it all pointed to the PCB so I thought, sod it, I'll just fit one. After all, that has happened twice on this boiler and the last time it had a PCB was (according to the date stamp on it) June 2001. Anyway, I bought one, fitted it and away it went - fired up after a few seconds and kept going. Working fine now, time and time again so hopefully that's it sorted. For now. The new part cost me £188 including vat. I could have got one a bit cheaper on the 'net but that would involve a few days wait for delivery, or even a 'reconditioned' one for about £40 but there were a lot of warnings about using genuine OE parts so I played it safe and bought one over the counter from a major heating supplier on my way home. Took about 20 minutes to fit.

One minor balls up. While I had my heating engineer cap on and feeling full of confidence, I thought I'd get the air out of one of the radiators in the kitchen. Only problem is that it has a blocked/knackered bleed valve so I thought I'd just loosen the blanking plug on the other side of the rad. I loosened it. Loosened it a little bit more. Loosened it some more and WHOOSH! The damn thing shot out followed by a fountain of what looked like squid ink! I stuck my thumb in the hole while I picked up the banking plug and put it back in. What a mess! Two bits of luck - the water was relatively warm so I didn't burn my thumb and it went over the laminate floor (would have totally ruined a carpet) so I quickly mopped that up with a couple of the dogs towels. They are in the washing machine as I type and they are going straight in the drier and back in the cupboard before my wife gets home from work tonight - she's a typical worry-guts. After I fixed it she saw the dog sniffing the air and swore it could smell gas!!

I think I'll get a service and a powerflush in the new year. And some heating insurance! Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

She's caught me! She's noticed 'black stuff' on the washing machine, bread bin, top cupboards, wall and chair leg so I've had to 'fess up. It's like being married to bloody Columbo! Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

Past master wrote:

If the boiler is 20 years old it really is time to get it replaced.

If you put a new boiler in make sure the system is fully drained, flushed and cleaned first as it sounds as if your system could do with an overhaul. I replace the inhibitor every 5 yrs with ours, with a flush of the system for a couple of hours at the same time.

I replace the inhibitor every 5 yrs with ours, with a flush of the system for a couple of hours at the same time.

Why?

I can do it myself so costs me nothing but the cost of the inhibitor, plus we have UFH. I would recommend that to anyone as it will prolong the life of the boiler/heat exchanger, but perhaps if you are getting someone else to do it then you may need to look at the cost/benefit of this.