Thank you all for your replies. Jack and Stewpot, especially. Jack section 2 was the original front chimney stack that was removed, section 1 is original and section 3 has been pushed up and along by the alternative slate. The roof is 1930's but sourcing reclaimed slate around here is not hard, there's loads of slate. The valley damage was created by the downpipe bowl having too much weight put on it, splitting it (joint asset) and the bottom of the valley being stepped on causing the split to the valley. The contractors that did the repairs on the neighbours side, left slipped slates in the damaged trough from the side they were fixing and from ours. The neighbour has had contractors on the roof, joint areas, without prior notice twice and you would imagine that a decent contractor would have taken pictures of pre-existing damage to cover their liability.
Building Control have rules about "In keeping" with the original, I was hoping that someone had experience or advice. Stewpot you're right only the lawyers will win in the end but I'd like to think that I might not have to live the eye sore as a reminder of how strangers damaged my home.