Thanks for your replies, Fleecewife and Twizzle. Sorry for not replying earlier but I had a computer problem, so I was unable read them until this evening. I have been checking him several times a day and the core has dried completely. I did spray around the base with barrier blow fly repel the morning after he pulled it off, and, fortunetly, we had a couple of wet, windy days immediately after it happened- which has kept the flies away.
I looked at summer fly cream in Wynnstay last week, I think I will buy some tomorrow, as a useful additon to the meds cabinet I think you are right about how he keeps doing it Fleecewife. He is a real character and into everything. Dylan comes running when he is called ( like a dog), spends half his day browsing the hedges, on his hind legs,( like a goat) and generally pokes his head into anything that looks interesting and could possibly result in a mouthful of food! ( pockets, bags, wheelbarrows, etc). One of my coloured Ryeland girls did try to copy his hind leg technique, to reach some new hawthorn leaves, but it was 'a bridge too far' for her little, dumpy legs. Maybe it's a Welsh Mountain Sheep thing. So far this year I have had to release four of my neighbours welsh Mountain lambs from the stock fencing behind our house, after they have got their heads stuck, eating the grass on our side of the fence, even though they have tens of acres to graze on their own side!
Hopefully he will only ever pull the sheath off his horn and not the whole horn. That sounds a bit drastic! Thanks again.