Oh yeah, *and*...
The leak from the shower onto the stairs was Not Fixed by my optimistic efforts with the silicone sealant gun. So it's probably leaking from the waste pipework rather than the shower tray (as my efforts with the silicone sealant gun were fairly robust). So we might as well move house. It's that or take a claw hammer to the ceiling above the stairs. Either way, it's going to be grievous. Arse.