It's amazing what you can achieve with an allen key. I didn't have much time at the weekend for mucking around with bike repairs, but I did go to the Spittalfields agricultural show (yes, it's the London event of the year!) and there the kindly folks from the London Cycling Campaign were in evidence. They showed me around a derailleur and gently pointed out that the whole apparatus could be "swung" around the frame, and that possibly my derailleur had simply slipped and that was why it was scraping against the chain.
Sure enough, early this morning I got to do a bit of fiddling - and lo and behold the problem seems to have miraculously gone away! How amazingly satisfying to feel that you've carried out a (albeit minor) bike repair on your own, not spent your money, and got a good result.
The words of the Jabberwock were on my brain when I cycled down the canal this morning:
"Oh frabjous day, callou callay!"
I haven't quite slain the Jabberwock - but I've moved on to first base when it comes to repairs!