Humm I do think he's taking the p a little, but then again he does have every right to be annoyed. Personally I'd offer to buy the wood myself then you'll know how much that costs and throw him a few extra quid to cover the labour for repairing it. You need to keep relationship well with him cos there's nothing worse than arguing with neighbours. Also to avoid any future problems would it be possible for you to put some electric fencing in front of his fence line then at least they won't do it again?