I do not know what happened during you service ,which one was it you don´t say or what type of 3 series you have.

Things to look at there are on some models a small air tube attached to the air intake you cannot see it from the top and it is easily forgotten if they have replaced the air filter.So is the MAF lead easily forgotten or not properly seated.If you car car has Vanos then this can cause alsorts of problems if not properly replaced.The list is endless they are terrible engines to work on not user friendly sensors all over the place and some are almost impossible to get at without removing halve the engine parts andyone could be faulty or not properly plugged in then it could be something completely different like binding brakes.

After one service, I found that the heated rear screen didn't work. I checked the fuse box and the fuse was blown. Strange, that. Perhaps I should have gone back, all innocent like, and listened to what story they would tell me as to why it wasn't working, but I put a new fuse in and I haven't been back there. Just a thought...

Replaced after market air filter with a OEM air filter and all is now normal, MPG has returned to before service level

Thanks for your replies and help guys