LEAK UPDATE: OK, so the drive home was through some torrential rain and I was expecting the same paddling pool effect/water feature under the passenger seat when I got back. But no, no water, just a little damp, but no worse than when I left Wales. (A fan heater is currently in the car drying it out).
So whatever it was, either Mrs Keyser fixed it with her investigation under the cover of Moreton reception, or some dumbass decided to drive through a Norths Wales monsoon with the passenger window ever-so-slightly open
We will never know....