Well, for the benefit of anyone trying the same thing…
The natural yeast got going pretty quickly and was happily fizzing away, but the smell was not right at all. Possibly there was nothing wrong – I don’t know – but not wanting to end up with 20 litres of chain degreaser I chucked in some campden tablets, which slowed it down a bit.
Some packets of yeast arrived on Tuesday, so I put one of those in. It now smells a lot more sensible. A sort of yeasty, slightly alcoholic grapey smell.
I chucked in some more grapes that I found on the vine, which is why it looks like alien brains.