Ascertain that we need to fit a proper mounting box for the new switch, since the existing one appears to be wood (and a very non-standard size, which was the problem in the first place). Existing box is too small so there is some level of chiselling away bits of wall involved.
Chisel bits of wall away, discover wires running through metal conduit, figure out how to get conduit running into wall mounting box (with aid of Internet), read about 'flying earths', read about old lighting cores without earth wires, swear. Try to find bit of spare cable to run flying earth between mounting box and metal switch, as yet unsuccessfully. Swear.
I hope this doesn't mean we need to get the place rewired because it would be a monumental pain in the arse, not to mention yet more expense. And I've no idea how we would actually do rewiring in some rooms given that you cannot take up the floor above them (the hall, as the floor in the library would not come up; likewise now the kitchen, as the floor in the bathroom isn't going anywhere in a hurry)