The breather pipe that goes into the intake pipe is connected to the head cover (not the head itself). It is there to let oil vapour to be sucked into the engine to be burnt off before being vented to the atmosphere. It's a requirement by law, and in theory should fail an MOT if it's not connected unless plumbed to a catch tank. However, your car should run fine without it connected (put a bung in the hole in the intake pipe though). Some cars will vent a bit of oil as well, though I know mine doesn't (I have a filter instead of the pipe and it's clean).
Remove the pipe just to confirm it's not that. Then check the other usual suspects - plugs and leads, check the condition of the intake pipe, reset the ECU. ... other than that, dunno
Mark