I would concur with Clubman. Get rid of the windows a.s.a.p. We had a similar situation (with trees causing potential damage, limitation on our property, even maybe loss of potential value) and it was messy to sort. Our 'neighbours - the 'most respectbalbe people on the road' stonewalled us as every point, and it got messy in the end. My point is that in my experience if you give someone an inch they take a mile, particularly if it is to do with their own property, they think they can do what they like. Alos, local authourity (council) enforcement is a VERY slow process, so get working on it straight away. Same neighbours tried a similar windows trick as yours as well, but we got it sorted in advance by pointing out to the planners that no property was supposed to be able to overlook the 'rear curtilage' of an adjacent property where it had not been the case prior to that. We got out town development plan from the council and referenced that, you may need to do same. Good luck, it will create a bit of bad blood whatever you decide, but they have started it by being completely inconsiderate of you and your property's value, future viability etc.