Solved

For a few weeks I have had an ache in my wrist. Now why I did not immediately realise that it was as a result of all the computer work I have been doing recently, editing photos, making photo books, writing up research, I don't know, as I had the same thing years ago when I was working and had to spend a lot of time writing and editing. Then I had a wrist rest and it helped a lot. 

Eventually the thing disintegrated and by then I was doing much less at the computer so I discarded it and thought no more about it.

A few days ago I came to the realisation that I was suffering the same as previously, so I looked for and bought a new wrist rest and it is fantastic. I can use it with a track pad or mouse and it works a treat. The gel cushion is a joy to use and I have no more pain. (I know it's lots of nasty plastic stuff, but needs must I'm afraid. And it will last me along time.)