Another update! (sorry for the delay)
Mrs scremmet went to the vet, had a chat, and the vet came out later in the day to check over the flock.
It was, indeed, orf and it is apparently ok to spray it with terramycin.
We've been doing so since then, along with a few tiny sores on some other sheeps mouths and it's subsiding quite nicely.
We've also bought four hurdles and for the time being we're checking them over with a fine tooth comb every evening!
Scremmet.