Stock training is relatively easy in paddocks and small fields, its just a matter of reinforcing the commands. its a different ballgame when up on the moors and a single sheep 'bolts' out of cover. i have seen well stock broken dogs treat them a wee bit 'differently'.
Robert if your pup isnt mauling them, but just chasing them, take it somewhere full of sheep and as the other posters have said, punnish where neccessary, even the dumbest of diogs will soon get the message.
I hope all turns out well for you, keep at it.
Yours in sport Pernod