Generally speaking if your megane is misfiring(or firing on 3 cylinders) it is due to a coil failure. These cost about 35 quid, plus vat, to buy. Each spark plug has it's own coil. That's a renault thing! To find out which on isn't working is quite easy on my type of megane - 1.4 16V. I don't know about the other types on engines
What to do is - open bonnet, on top of the engine there is a plate. this is just held in place with clips, so pulls off. Under the plate you should see four grey round caps. These are the top of the individual coils. There should be leads connected to each one. Start engine. Remove left hand lead, if the engine sounds worst, that one was working fine, so replace lead, and move onto the next lead. The faulty coil will not make the engine sound worse when It's lead is removed. Once this has been found, it can be removed using a torx screw driver and replaced. That should do it.
Roy
July 2007