Can anyone tell me how much MPG can be gained by a remap like the one suggested here
http://bellautoservices.co.uk/bas-remap-interface/
and another mod suggested here
Www.Superchips.co.uk
and is it likely to affect an approved used warranty?

if they fine out your running a chip, then yes it may effect the warranty

Also, some find they improve mpg, but most say they just increase the driving pleasure through the gears Former Mod/Member, with the most post & Chicken George Arch nemesis

MPG improvements are 10000000% dependant on your driving style, most companies claim 10%
In my FL2 (superchips) I didnt get any savings because it made me want to drive faster because of the flexibility of the engine.
A remap usually means you have to change gears less and change up sooner saving fuel that way. On my Vw passat driving sedatly and changing up before 2000rpm I am getting 53mpg compared with 45mpg before the remap. I used a company called Carbon Chip Tuning in Altrincham. plus when I want to put my foot down I have 200hp on tap