I've cancelled (and turned punters down) plenty of times and not given a reason or made up an excuse; the only reason you need is that you won't be seeing them and as long as you let them know rather than having them turn up and blanking them I don't think it matters. All the cobblers about periods, landlords, hot water problems and so on just sound like a pack of lies anyway even when they're not.

If it's short notice (less than an hour or two, say) and I want them to rebook another time I just offer them twenty quid off or something .