OK, now it's not raining, I've had another look and taken a better photo (attached). She's also limping a lot less this morning, so my hope is that this was an abscess which has now burst. The toe of the hoof has a pocket (actually it's sort of missing, and no I didn't cut it off!), so maybe something has got pushed up inside there to cause the problem.
Penninehillbilly - yes, of course you can join in. The rules are that you need to scratch your head about a problem for at least a week, and only then post about it on TAS.
Within 12 hours, new information will come to light which will make you look as though a) you haven't got a clue what you're doing, b) you didn't check your animals properly in the first place and c) is probably nothing much, but could be something far far worse, which you will then worry about incessantly. (
CODD is a relatively new disease to the UK, which looks a lot like this, with infection around the top of the hoof which then tracks downwards, and is really, really nasty once it takes hold in a flock.) Good luck!