I hope its just the tensioner and is easy to fix, they are lovely cars when in good working order.
If you haven't got one already, one of these cheap little cameras should fit down the sparkplug hole may have enough resolution to confirm any contact marks on pistons.
Also worth scanning with VCDS to see what error codes come up, and to see if its recording any actual missfires.
http://www.ebay.co.uk/itm/USB-Waterp...cAAOSw~gRV1BSs
http://www.ebay.co.uk/itm/2m-High-Re...8AAOSwFnFV~21H
Let me know if you want ElsaWin instructions for tensioner testing, I can easily copy it to PDF