honda civic 1.8 petrol (uk model)  started to  overheat and misfire took it to local garage, mechanic said head gasket was OK, the was a small coolant pipe leaking at the back of the engine he fixed that, so he replaced the small coolant pipe, one ignition coil with news ones, car was ok first couple of times (couple of 2 mile trips) at first it was small misfires now it's much bigger misfires, (I'm thinking maybe spark plugs but Installed brand new spark plugs before the overheating issue and I assume the mechanic would have checked rest of the ignition coils  apart from spark plugs and ignition coils, what else could be causing the misfire