I realise my problem is shared by many others and maybe folks are weary of explaining the same advice. I tried to see if there was anything on the Apple Support discussions and, of course, there are quite a few topics on the same theme. Strangely, when I tried to login to post a question, I seem to be barred from seeing any of the discussion, with the links all being 'out of date.' I found the discussions again on my iPad, tried to login again and was barred again! So clearly Apple are not wanting anyone posting more comments about how awful this update is.
One possible way of fixing it was to do a reinstall of Yosemite but I can't find how to do this. Can anyone explain how this is done?