This mysteriously resolved itself a few days later. No idea what the problem was but I did have a problem with corruption of a folder or two recently. Think it was a problem with a sector of the hard drive, so perhaps other files were affected, causing this issue.

Anyway, over now and non-reproducible [irreproducible?]

OK, this is a weird one. I created a new identity a few days ago. I based it on the default identity. I then decided to make the message reader/preview font different to the one I normally use.

What I've noticed today is that this one identity - only this one - will not re-colour and italicise the quoted text in messages. Viewing the same messages whilst set to a different identity brings back the differentiation. What on earth could cause that? Thinking caps on...