Well, the plot thickens!! She was doing a three-legged hop this afternoon, so I went to trim away the detached hoof as suggested. However, I noticed a very slight swelling and puffiness at the very top of the hoof, on the worst side. It wasn't there yesterday, but one foot is noticably different from the other today. The vet thinks this is an abscess starting under the hoof and up into the foot, caused by some of the gunk that's been impacted up there. I've picked up some Alamycin for her, and we'll see what happens next. At least that would explain why she went so lame, so quickly.
So Deere, it looks as though you were right all along!

Thanks for all your help everybody - as a sheep novice, it's lovely to be able to discuss this sort of thing with friends, even virtual ones. It just makes dealing with these issues for the first time so much less daunting

.