you say you have a standard system- before changing anything why not remove your center silencer for £50 and see how you get along.
i did this and its one of the best mods i have done, so in love with the sound i get now from my car.
for anyone who has the front left knocking- i just had my top mounts and dampers replaced in the front. (dampers replaced to be on the safe side)
the top mounts were completely perished .
i did the work at Beaniesport who told me in the last few month he has done about 4 and it must be age...
i also had that once or twice in my 4 year ownership and was a simple mis fire- but nothing needed changing or repairing just a misfire. only real way to know is to reset all your codes then wait for it to happen again.