Fabia vRS still not happy

Okay, so the oil leak is fixed (gasket on rocker cover) but friendly mechanic has had to give up on trying to fix my car. He just doesn't know what else to try. He doesn't think that the injectors are the problem as he disconnected them one-by-one and it made no difference to the running. He also couldn't work out why it won't start properly. He's very frustrated cos he wanted to learn from it, but he is at a loss.

So, where to go from here? I currently have a 5-year-old car that doesn't start properly and has poor fuel-consumption. I reckon it'll cost me an extra £250 of fuel per-year to run if I leave it as it is, but how much more will I have to spend on it to get these problems fixed? And I don't really want to buy a new car right now. Not in a year when we've done new bathroom, new windows and new boiler already!!!