Quick update - I cleaned it up and had another look underneath. The clutch pulley system is all working fine, but the belt appears to have turned itself inside out. As the belt has a sort of a v cross-section, I imagine that the thicker edge of the belt is now within the pulleys as it travels, therefore giving much more friction - even when the clutch is depressed.
Does that sound likely?
anyway - I have to figure out how to replace both belts - doesn't look obvious or simple, but mabye even I can figure it out!

Last edited by CaptainCranky; 03/04/14 02:53 AM.