I'm guessing my car must be the one that is for sale in this thread from this forum:
http://www.306gti6.com/forum/showthread.php?id=119564
According to that, engine made a knocking noise when revs dropped which fits with what you have said. I have contacted Richard (Pugplus) about it today and he is going to have a look on his computer later to see if he can give me anymore info or history about it.
Seems to be running spot on to me though!
Only reason I asked about head gasket, is because I started a thread in 'Engines' section a couple of weeks ago, because my car hesitates, almost like a misfire, for the first time you put your foot down when it is started from cold. Then it clears and is fine from then on. People on here said change the green temp sensor, so I did and it still occasionally does it. So the guys on here then suggested it might be early signs of head gasket failure. But according to the thread above and what you said, the head gasket has been done 18/24 months ago. Although that's not to say it couldn't go again, but it doesn't seem to be losing coolant or showing any normal signs of head gasket failure, so perhaps my problem relates to something else.
Interesting stuff to know anyway.
Cheers