F is for failure to light, the main reason usually being the igniter and flame sensor, which can be replaced.
Parts can be bought from Parts Center (note the spelling, part of Plumb Center) and remember to include a combustion chamber O-ring seal.
Keep a spare of each for next time, usually 8-12 months.
Fitting details are very well explained in the maintenace manual, a copy of which should have been supplied with the boiler.
You can do this without a Corgi man because you're not touching the gas supply, it's all mechanical.
Check the fan rotates freely as well.
If you still have the boiler and want more advice call me on 01723 890556 after hours.
Ray Milner
January 2008