My flat is fitted with switchable sockets as per below:
Please settle an argument a friend and I are having: if a socket is switched "on", is it wasting electricity if nothing is actually plugged into it? If so, any idea how much?
No. With nothing plugged in there is no circuit, so no current can flow.
An exception to this is if the socket or the switch has an indicator light - usually a neon one - that is illuminated when the switch is on. In that case the light will use a (very small) amount of power.