The rotten eggs smell will be your cat. Thing is cats don't just fail. The missfire has probably caused it to fail and replacing your cat won't solve the problem.
The missfire is probably caused by a faulty coil pack or the plug leads but you would usually get a wet plug on the failing cylinder. When the engine is missfiring pull a lead of one at a time. The engine should almost cut out if that cylinder is still firing (because it will only be running on at most 2 cylinders) If the engine doesn't change then you know that's the faulty cylinder.
My apologies to everyone for not posting that I had fixed the problem.
I found a broken wire going to the coil pack (The far right of the 3 as you look from the front) it had been repaired before (Soldered by previous owner) I resoldered it for now until I get the chance to fit a new block to it.
Problem solved !
Thanks for the reply.