Thanks for all the answers. To confirm a couple of things:
1. Became aware I was gripping the bars fairly tightly, but the decents at Afan on Whites especially were so rocky felt I couldn’t loosen up much more than I was, I was consciously trying to relax, but the pain was still there.
2. I was braking with 1 finger as suggested. However I was keeping it floating over the lever for most of the way down.
3. I have recently adjusted the angle of the levers to be a bit more ‘up’. At suggestion of Jedi on a UK Bike Skills course, to help keep a natural break in my wrist. Maybe I adjusted it a bit too far?
Was thinking to change the Ruffians for a set of Rogue grips, or should I not bother? The powerball thing sounds like it’s worth a try. Unfortunately I live in Norfolk, not the hilliest of terrains, so can’t get rid of the problem by riding more long decents, as we don’t have them :D