In view of all the positive replies on ccleaner I decided to go for it.
It messed something up which prevented Firefox from displaying the BBC site properly but after a bit of tentative playing around (Restoring, re- installing Firefox) I accidentally discovered that by clearing everything in Firefox-cache, history, and cookies- it was ok.
I do have a question though, after running ccleaner yesterday I used the analyze feature again today just for interest, and there is another 12 MB or so of new "crap". Is this a normal consequence of just browsing around the internet?