They've been doing this in Sark for many years.
Looks like they chase the ones in Moffat however, which seems a bit unfair. In Sark, they are herded gently into the start pen, with their "safe" trailer at the other end which they are used to and means home, so when released it is not stressful to run for home.
Good clean fun and the sheep don't seem to get too stressed at all, certainly no more than sheep dog trisls