on dual mass systems the aux drive pulley on the crank has a "rubber" or clutch type dampaner to take out the vibrations / shock to the aux drive belts to give smooth delivery to the belts and avoid "snatching" . when they go basically the dampening bit in it poo's itself and you get the knock from it . simple to change . always advise to change the aux drive belts while you are there .
Pulley might take osme shopping , example is here : http://www.ebay.co.u...=item2c6372f144
Its not mine but you get the general idea of what you maybe after .
Thats if its deffinetly the pulley , maybe the dual mass flywheel etc . worth a proper diagnoisis at the garage maybe as opinions are like assholes on the internet - everyones got one . Even me 
if that is what the fault is , maybe an hour or two to remove and replace along with the belts tops according to book times .
That's the thing, the car was took to a few garages and we were told different things by each garage and one garage said they'd replace X part but wouldn't guarantee it'd fix the problem, which to me is a complete joke when you're spending hard earned money on something that was fine!
And I may join the Renault forum, just that I'm more active on these forums and I know that there are few mechanics on here and people who knew a bit about cars, doesn't hurt to ask!
Main problem with the car now is its SORN, no tax, insurance or MOT so it's the problem of getting it to the garage

It's a shame as it was/is a very nice car inside and out, but after reading up more after the post and watching some YouTube vids, I think it's safe to say its the dephaser pulley thing, next job now is to find someone to fit it!