Looks like you want the Animals act 1971
http://www.thekennelclub.org.uk/item/1052
http://www.opsi.gov.u...9710022_en_1#pb1-l1g4
3 Liability for injury done by dogs to livestock
Where a dog causes damage by killing or injuring livestock, any person who is a keeper of the dog is liable for the damage, except as otherwise provided by this Act.
4 Liability for damage and expenses due to trespassing livestock
(1)Where livestock belonging to any person strays on to land in the ownership or occupation of another and—
(a)damage is done by the livestock to the land or to any property on it which is in the ownership or possession of the other person; or
(b)any expenses are reasonably incurred by that other person in keeping the livestock while it cannot be restored to the person to whom it belongs or while it is detained in pursuance of section 7 of this Act, or in ascertaining to whom it belongs;
the person to whom the livestock belongs is liable for the damage or expenses, except as otherwise provided by this Act.
(2)For the purposes of this section any livestock belongs to the person in whose possession it is.