All true, and the OP is apparently going to discuss all this with his mother. However, it would seem that he feels her inclination will be to 'let things be', doing without the intercom, in order to avoid the hassle and potential cost of opening up a potential can of worms. It's clearly not what we would advise, but it's ultimately her decision.I cant comment on what it would or wouldnt cost to get the DNO in, and that might be the best first option but I cant imagine it would cost and arm and a leg to get a spark in for an hour to give it a look over and shed some light. Some might be able to do that job on there own, but where the other wires go, and where the feed for the flats comes to is potentally relivent.
Almost anything is possible!Although thought that comes to me is, is it possable that the locked box contains a meter? Seems unlikely but so does the shared lighting being unmetered.
Kind Regards, John