I can empathise.... my two are also wee escape artists if the grass is greener!!

Personally, I think he is over reacting over the damage to his land - if there were already animals on it, as opposed to crops, they will not have made it any worse than the cows potentially could have (I have had next doors cows escape into my hay barn and lunging area and boy what a mess, but I also realised the farmer could not have prevented it).

I think £150 is totally unacceptable. I would not ask him to fix your fence, sort it out by yourself, and give the farmer a bottle of wine/whisky as way of an apology. The money would not go directly towards repairin damage to his field!!!