You potentially have a claim in common law negligence because the farmer has failed to keep the livestock under control.
It will help if I explain how claims in negligence arise.
There needs to be
1. a duty of care
2. the duty of care needs to be breached
3. as a result of the breach there needs to be loss or injury
4. the loss or injury resulting from the breach must be reasonably foreseeable.
In order for there to be a claim in negligence, all 4 heads have to be satisfied.
The potential stumbling block for you here is whether this is reasonably foreseeable and it’s probably not so I think there is a good chance that if this went to court, you would lose. Not guaranteed, but that’s my opinion.
Do remember that I can only give you my opinion.
Another lawyer may have a different opinion. Litigation needs at least 2 parties and neither goes to court expecting to lose.
Nonetheless, one of them does, even though they have been told by their respective legal advisers that they have a good chance of success.
If there was a black-and-white answer to every legal problem there would be no need for anything to ever proceed to court.
You can always ask him to pay the GBP500 repair bill but if you want, then your only remedy is indeed to take him to the Small Claims Court.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have