If they are saying it's the brake switch well that the first time I've heard that one , but go along with it the switch is £10 max, the other problem that throws up the F is apparently the gear selector module £270 + so lets hope it's the switch.
beause of the number of cars with the big 'F' problem I started a page with some information, if you can add positive information to that page please mail lofty with details .
Bert Rowe's-A-class info. Gearbox (Manual)
and link to auto & acs gearbox top of the page
