I think I've located the wasp's nest. My study has a vent connecting into the chimney; as this was my prime suspect for the location, I taped clingfilm over the vent. As of this morning, there are a couple of dead wasps stuck between the vent and the clingfilm, so that seems pretty conclusive.
Only a little BIY today; 24 hours duly having past, I could go back and grout the restored tiles. It took rather longer than I was expecting, but I wasn't sure that I had enough grout so I had to restrain my profilgate tendencies and do things a little more catiously. Finished that just in time to go out dancing for the first time in a while. I'm still feeling very rusty (only been out half a dozen times in the last couple of years), but at least I seem to be slightly fitter than the last time I did it.