Bear'smate thanks for that! I have only come across it in the instance where someone's dog was walking ahead of her offlead (not worrying livestock but not on a lead) and the dog was shot.
I was informed then that it was the farmer's perrogative so have always been extra careful. I remember being told not to put them on extending leads around livestock because farmers can't always see the thin line and it looks like the dog's offlead.

Anyhow - thanks for the info, but if farmers will still do it (as has been seen by moobli's example and the example I remember) I think I'll still stay careful! An arrest on the farmer won't bring my dog back.