Thanks for the ideas - as it happens I have just bought a new halyard and a new block. I'll let you know if the old spinni behaves itself with these in place.

Just for info, the spinni head block is on a swivel, suspended from a foot or so further up the mast, but was showing signs of wear in the side plates (on the inside faces), so given your input I now think the halyard most likely was not swiveling as it should have.

Cheers