Good neighbourly relations applies to the neighbour who attached his fence to wrjelly's fence also. He should have asked permission.
I have a friend who was holiday some years ago and returned to find his neighbour had erected a concrete post and arris rail fence between the gardens with the posts cut into my friends lawn. Needless to say he had to take it down and re-erect it with the posts on his property.
Yes Dodger, you're absolutely right. The point I was trying to make is that making a "legal" fuss about it would probably end up with bad relationships for a long time.