Not sure of the finer points of this, don't think any 'law' has been broken, would be a small claims damages issue.
But......... someone else put them in to the field with the ewes so you would have a defence.
Even though de fence was broken
Exactly. This is a storm in teacup bit of nonsense... sheep do/will escape and some well-intentioned 3rd party did the 'wrong' but reasonable thing. While it is upsetting for the guy who might/might not have had a closed herd it'd be all but impossible to make a claim against anyone. So it should be a case of folk sitting down together, apologising and figuring out a compromise.
The closed herd owner had as much responsibilty to have his stock gates padlocked and signed as did the one whose sheep escaped.
Escaping sheep happen all the time around here..and unlabelled, escaped sheep that you can't track explains half my tiny flock of pets (two made it here and we couldn't trace owners so they get permenant sanctuary). I've traced and had at least a dozen collected from my property over the last 2 years. Some come via the front gate, some wade up the streams, some find new holes in fencing. You can't find every hole in 2 miles of fencing every day..specially when half the fences are't mine.
daftest of all is that to call trading standards and get a flock number address costs the caller money...so no-one rings. If neighbours dont recognise the number or no-one comes looking then - well you can guess how easy it is for tags to get swapped. Most of the farmers round here only have a rudimentary idea of how many sheep they have anyway with the dead, missing, drowned and washed away etc.
It's easy with my few pet sheep.. we can see them from the house and go give treats and a pat twice a day.. and if they did escape it'd be straight to the house for rosebush.