Thanks - I hadn't checked the cool-er disc... good idea.
I did try a different version of the firmware (I have the last two versions archived), but it didn't work. So then, I used the latest software update again, and although it didn't flash in the way the instructions advise, I carried on... and the update seems to have worked. My cool-er is back in working order for the moment.
Interestingly, I did look around at all the other ebook readers available now... and the only one that seems a good alternative is the kobo. But I also use my iPad as an ebook reader, and it's brilliant - just a bit heavy, and too large to carry in a handbag.
Thanks for replying.