I didn't realise fault codes could be cleared from the system...why would the recovery services do that when they're reporting that further investigation is needed? Like you say, that'd just handicap the people who need to know to be able to diagnose the problem.

I sort of half suspected that the replacement battery I had put in a month or two ago was actually not connected up properly...but the garage tells me that wouldn't have caused the non start or the buzzing noise.

The only thing to do now is carry a spanner in the car, write to Ford, and if it happens again take it back to the garage and leave it with them until they can replicate the problem.