Thanks everyone. Our temporary solution is to stop using the main bathroom!
At some point this [What do we call it? Postmas? Twixmas?] I will go shopping for sealant from Wilko. One thing I can do is wield a sealant gun! Although, if the leak isn't due to the sealant around the bath, it'll be a professional job: the entire bath side panel is tiled to match the walls and completely grouted into the surrounding walls and floor, so even if we were to bash it off ourselves and by some miracle know how to re-seal the waste pipes, we couldn't put it back.
We have our small en suite shower room that everyone will use. It is also getting a bit old as it was the first room we did when we moved in; used to be WOW SHINY but now is in need of some small remedial works (paint, re-sealing just-in-case, managed to chip a tile, showerhead-holder-rail-thing is wobbly).
But meanwhile! Merry Christmas Eve to all!
We're going to roast all the veg and potatoes this evening, stick them in overproof serving dishes and cover with foil to reheat tomorrow. Must remember to take the braised red cabbage and the pigs in blankets out of the freezer.
DH also wants to make a Yule log today, because two different flavours of Christmas pudding, several boxes of mince pies, a panettone, 4 bags of Lebkuchen, 2 bags of Pfeffernusse, a home-made boozy Christmas cake and a huge gingerbread house just aren't enough sweet things. I may put my foot down.