Exactly the same thing happened to me after an mot (to a brand new alternator), on the way home I stopped at a petrol station (a busy one at that), filled up, went to start it and it wouldn't! Turned out the battery was flat and wasn't charging, and all the electrics had been running off the battery from the mot station to the petrol station.
Guy behind beeped at me thinking I was being a tw*t who takes ages to pull away from the pump and I got out to push and I think he thought I was getting out to nut him, he looked terrified
Anyway, got the car home and turns out the mot centre had banged it on the ramps coz of the stupid place it is on the si and broken it
Took it back the next day but the mot centre said I cudn't prove it was them, so I ended up ditching my brand new alternator and getting one from a scrappy for £30, and have been running this one ever since
