Just paid an electrician to check our lighting circuits. I’ve been puzzled by a flickering hall lamp (Compact Fluorescent type) when the light switch is in its off position ! Silly of me I put a meter on the socket and discovered 88 Volts with the switch off. But zero on other light circuits around the house.  My concern was compounded by recent heating work and the possibility of a water leak, as we’ve just pressurised our system. It turns out if you Google this, that this is a fairly common problem on stairs , double switched lighting systems. Evidently the extra cabling/capacitance creates sufficient coupling to present enough volts to make some CFL lamps flicker. So now you know.