I realise now what may be the cause.

Shame, never blown a chip in 20 years of cmos etc

I originally had a worn leather executive chair at the bench, it always annoyed me as it was just too wide for the opening and prevented me opening the work drawers unless I pulled back. I recently changed it for a typist style chair with arms which is narrower, now I can open the drawers without moving back.

Unfortunately two things:-
one: it has a synthetic cloth seat and plastic castors.
two: I bought a plastic over carpet mat to help movement.

Seems these two are perfect static generators. I use a self healing craft mat on bench top, I suppose I'll have to invest in an anti-static mat and a wrist band now.