Sorry Caramba, but I must disagree with you about planting leylandi - v.dodgy, and a sure fire way of making enemies out of your neighbours.
The neighbours however have no right to prune your tree other than the parts that overhang their property (and then by law they're meant to return the cuttings to you - otherwise it would technically be theft believe it or not). Unfortunately, as with so many problems of this nature, its very difficult to settle the issue amicably and you obviously don't want to fall out with your neighbours. Have you thought about putting a fence up to regain some of your privacy?