Thanks, Peter, that's very helpful.

I'd found instructions on the web for removing the key levers (and extracted the offending one - the C# just above middle C)and had reached the conclusion that it was a tightening in the balance hole area.
There is a small protruding sliver of wood there which I'm going to gently remove with a parallel reamer of some sort (I am an instrument maker, just not keyboards, so have a fairly comprehensive workshop tool selection).

I'll also try cleaning the pin as you suggest. The other keys are fine.

I'm intrigued by the construction of the tangent end of the levers - there seems to be an insert of a chalky substance in the wood for about the final inch giving a kind of wood - "chalk" - wood sandwich layer effect. The chalk protrudes from the sides a little and rubs against the inset chalk of its neighbour.

I should have said, BTW, that this is an 40 year old entry-level Morley, not a really valuable historical instrument I'm messing with!!

Thanks for your suggestions.

Paws