Rosemary, the legislation used to be Section 1 of Dogs(protection of Livestock) Act, 1953 which has probably been amended by The Wildlife and Countryside Act1981.
" If a dog worries livestock on any agricultural land, the owner of the dog, and, if it is in the charge of another person other than the owner that person also shall commit an offence."
Worrying means, "attacking livestock, or chasing livestock in such a way as may reasonably be expected to cause injury or suffering to the livestock or, in the case of females, abortion, or loss of or diminution in their produce, or being at large (that is to say not on a lead or otherwise under close control) in a field or enclosure in which there are sheep." the exceptions you mentioned above apply.
The Act goes on but as you can see an offence has clearly been committed. Over to the police.
JD