Thanks all - waiting to hear back from WCC - will let you know how I get on.

neb: More specifically, get a convex mirror, put in interruptor in front of it (e.g. a spinning fan), and focus it at their bedroom or somewhere similarly intrusive to them. That way every time their lights come on, they get a high-intensity strobe directed into their house. And since it's their own light (your contribution is purely passive), there's nothing they can legally do about it.

Remind me never to upset you!

That is super evil genius!