Here's a suggestion... It may not work, just a suggestion..

Get your own code-reader (about 45 quid from a car-parts shop). Plug it in and find a technicaly competent friend to sit in the passenger seat and watch it whilst you go for a drive. After all, it'll cost you an hour's labour to find a garage mechanic who's willing to do that. It may show up something..... But first and always, look under the bonnet and just check that everything is as it should be. Is the air filter clean, for instance? Little things can upset the miriad of sensors in a modern car.

The thing that bugs me here is.. if the engine management light comes in, there should be a fault code! And if the engine management light goes out again, the fault code should remain stored and readable! I find it extraordinary that the light has been on, but the garage says that no codes have been stored. Some contributors to this board are quite knowledgeable about modern car electronics.. perhaps someone can explain this?