Thanls for the advice Hugh, I had done that previously with no success, but it now appears to have worked!
The main difficulty in all of this is that it's so intermittent. I can sometimes get "scanner not found" on boot up, then another time it will be fine, and to make matters somewhat worse the scanner itself always performs OK, as does the software. This morning it's behaving again.
I had begun to wonder if the scanner itself was on the way out, but I'm reluctant to change it yet as I use it for slide and neg scanning, and those are not easy to find for good results.
Many thanks again Hugh, I really appreciate your advice.
Les