just had the MOT test done, picked a local garage that I have never used before, car failed on the rear side light, bloke said it wasn't working,

These are the LED type which I fitted in July of this year, I told him that it does work so we went and checked and he saw it on and amended the MOT to PASS .

BUT, I must admit, I got away with that one, as earlier today I tried the lights and the side lamp passenger side did not come on, it did come on with the ignition on.

I either dreamt this or read it somewhere that perhaps if the battery is low LEDs sometimes struggle ??? ie either flicker or stay off until ignition/engine started ?? Any ideas . . . .? 2017 SEAT Ateca 4drive 2.0 xcellence (May 2022 -
2007 HSE with HST bodykit in Zermatt Silver 169,700 miles (dec 2016 - May 2022 )
2007 Honda Civic Type S GT (2012-2017)
2002 Mini Cooper S (2008-2012)
1992 Honda Legend 3.2i (2003-2008)
1990 Honda Concerto (1999-2003)
1985 Honda Accord (1997-1999)
1983 Honda Accord (1993-1997)
1983 Mini Mayfair (1988-1993)
1974 Mini 1000 (1979-1986)

I’ve had a bulb with a dodgy filament tap it and it came on.

yes it could be worth changing the side light bulb, anyone know what part it is ? 2017 SEAT Ateca 4drive 2.0 xcellence (May 2022 -
2007 HSE with HST bodykit in Zermatt Silver 169,700 miles (dec 2016 - May 2022 )
2007 Honda Civic Type S GT (2012-2017)
2002 Mini Cooper S (2008-2012)
1992 Honda Legend 3.2i (2003-2008)
1990 Honda Concerto (1999-2003)
1985 Honda Accord (1997-1999)
1983 Honda Accord (1993-1997)
1983 Mini Mayfair (1988-1993)
1974 Mini 1000 (1979-1986)

I think that on the FL2 each lamp is individually fed by an FET in the CJB, and will not come on if a fault is detected.

LED lights have to have the correct resistive load to work.

I also think that if a fault is triggered then a indication is made to the dashboard.

(I haven't checked this, but worth investigating) FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

See if your local council depot does mot's. I always use ours for all the families motors. Local authority mot centres don't do repairs, therefore they have nothing to gain by giving a dodgy fail. You always get an honest mot test.  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.