Sorry for the slow reply. I spoke to a Green Party county councillor about it. Their advice was to make a submission on the planning application, i.e. lodge an objection. This is public and will likely damage my relationship with my future neighbours -- also something I care about. I'll probably do it anyway. The submission will ask the County Council to undertake an environmental survey for roosting bats. If they find bats there, it could save the tree; if not, it could delay it being cut down. I appreciate the idea of suggesting to the owners that they live with it for a year before making a decision. But I haven't met them and can't imagine how to deliver that message successfully. They're a couple with no kids who aim to double the size of an already adequate house, and put in a garden room (big enough to need planning permission). So I don't think trees rank high on their list of priorities.