for sheep you're going to need a county parish holding number - CPH number. This is the number you'll use to record sheep movements on/off the land.
you get the CPH number from the rural payments agency these days I think, which is just a centralised bit of DEFRA.... if you google the subject you'll find the phone number for the main RPA line.... you'll be on hold for a while.
You'll also want to get some movement license forms (they used to be carbon copy, I assume they still are).... your local animal health office will probably send you some for free.... or pick some up from your local market.
HTH