Have you checked over the rest of the lamb's body? It's possible it is biting its legs because it can't reach any other bits, where it could have been fly struck. Check in its fleece right down to the skin, especially at the rump end and between the hind legs underneath.
If it is flystrike then it's an emergency or the lamb will die - it will need the maggots to be killed then antibiotics to prevent infection, general support until it recovers, and flystrike prevention.
There is no truth in the assumption that Soays are somehow resistant to fly strike, worms etc - they're not. Although it seems early for flystrike, it has already been reported in various places, due to the clement spring weather we have had.
This is the most plausible cause I can come up with, but there may be other ideas coming