Terry, many thanks. I think I've fixed it.
ClamXav kept identifying a couple of files, which I deleted as you recommended. Eventually I discovered I could trace the files (non-tech user here!) and they turned out to be a couple of e-mails from reputable sources - or at least they looked like they were reputable! I've now deleted those e-mails and all is well.
When I think back on it, my experiments didn't reflect what was running when I went on to Oople. It's entirely possible that the Mail app was running, and I mistook correlation for causation.
There's an awful lot going on in the Activity Monitor - does the malware look obvious? I've pretty much left everything alone that I don't understand, but there were a couple of things running that I don't use any more, so they've been deleted. All is now as it was.
This is the first time in 26 years I have had any trouble with my Mac, and in the end it turned out to be no trouble at all. Oople is working fine too!
I'd like this to be a reason to upgrade from my G4 (now seven years old and still going fine) to a new MacBook, but I don't think it is! Thanks again everyone, all your help much appreciated.