(Original post by jacketpotato)
Its important to understand the distinction between criminal and civil law. Criminal law is the thief against the state; civil law is you suing the thief. They are separate legal systems. The police are usually not able to force the thief to pay you compensation, but you could definitely use a conviction for theft in a civil case.
It sounds odd that the police are not pressing criminal charges, have you asked why not?
In terms of your legal remedies against the thief in the civil courts, we are in the area of personal property law and specifically the tort of conversion. This is a civil wrong dealing with the situation where someone takes your property and you suffer loss as a result. Like most of the law of contract and the law of tort (civil wrongs), there is no Act or Regulation which sets everything out for you. Its common law, which means the law is created by precedent (previous decisions of the courts).
Clearly, if someone takes 21 sheep they are required to return 21 sheep and/or compensate you for any loss you have suffered; so your legal position is that you are entitled to be paid damages for the sheep that weren't returned.
The issue is proving how many sheep there were. If you decided to take the thief to court, you'd have to prove on a balance of probabilities (1) that he stole 21 sheep and (2) what loss you suffered as a result of his actions (probably the value of the sheep). Obviously this depends on what evidence you have, if you have a receipt or another written document that shows you had 21 sheep that would be pretty convincing.