The decision's been made for me, for a few days at least. Went to open the gate for Merlin to have his 2 hours in the second field (I need to rest the bottom two fields but the top two are too lush for him to be on full time, so he and six sheep are on a strip for a couple of hours a day until it's grazed down enough for them to move up) and noticed he was nodding when he walked - swollen front right fetlock, poor boy.
It's a little bit warm and a little bit puffy, but he's happy enough to walk on it and take his weight on it as the front leg while grazing, so I'll keep it cold-hosed tonight and tomorrow morning and then if it's not improved by late morning I'll get the vet. I can't see any sign of a wound, my best guess is that he's charged up the hill and put his foot in one of the two new rabbit scrapes I noticed last night.
Put the weight tape on him this morning, bang in the middle between 484kg and 490kg, so he's put a bit on. He's on less grass though, now that the bottom field is shut off ready for spraying - maybe 2/3rds of an acre shared with the sheep plus the abovementioned 2 hours on richer grass.
UPDATE: 2 hours later, the swelling has reduced and the limp is almost gone :o) Looks like it had only just happened when I went down. Fingers crossed with more cold water he'll be more or less fine by tomorrow.