Cheers for all the help! If the 16V has any value as spares / an MOT failure I'd seriously consider selling both and buying one decent car (don't want to start another 8V versus 16V debate - there seem to be enough of those, however for the record, it would be a 16V!!!

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I took wheels off both cars today and both have original VW shock absorbers fitted all round. Also spoke to the original owner of the 16V (I work with him) and he claims he's never replaced the springs or dampers since new. Don't understand what's going on here, perhaps 16V engine is slightly heavier so car sits lower?
Also noticed the 16V has bigger brake discs with VW callipers instead of the 8V's small discs and Lucas Girling callipers. Is this normal? Guess the ABS pump changed too as they look physically different.
Any ideas on the clonk noise? It happens at quite low speeds (5mph). All the suspension bushings on the 8V look sh*gged (fair enough after 130,000 miles) so am having a hard time working out if it's wishbone, roll bar or subframe doing it. Don't think it's engine mount anymore.