Leylandii trees have very shallow roots that spread out close to the surface and will fall easly during strong winds, if you do have property nearby then you would have a case, if the council can order farmers to remove trees for safety reasons i don't see why they cant do the same in an estate.
If the trees are close to you boundary, get a rotavator and rotavate your side of the boundary, if there roots are there they will be destroyed and you are well entitled to do this as they are well entitled not to do anything on there side..at the end of the day i presume relations aren't that good anyway and if they have a problem, you gave them a chance before you rotavated to cut them. If there are not bothered about what you think, why should you/mother bother.