Thanks for your replies, chaps - and my chrome cap is the same as TBM's one.
I tried my best to avoid any reference to knobs in my original post, as I knew where the thread might go :-)
Arwyn - thanks for your considered comments, and I suspect the disassembly you mention will be needed. I'll try giving a squirt of lubricant into the hole on the cap (can't do any harm) but I'm not hopeful. The car is off to Techniques next month for a service, MoT and a bit of end-of-season tweaking, so I may just leave it to them to sort.
Dave
