A big thanks to Monsieur Guffer for having a look at her today.
It turns out that the radiator has a leak on the top right plastic-looking bit and this might have been spraying coolant onto the battery and causing a short.
There was no sign of a petrol leak.
I'm going to look for a new radiator so any suggestions as where to get one from would be much appreciated.
Thanks again MG.
It was just the coolant residue on the battery itself which made me think of this. The leak is on the top NHS of the rad, a crack in the plastic. Under idle and it mostly just trickles but we saw a huge amount come out and I can imagine that under load that coolant would spray up. This spray would hit the lpastic trim which kindly seems to angle the spray on to the battery.
This is just supposition at the moment but putting everything together (including that the k-seal had stopped it for a bit) lead me to this idea. I think it best to deal with that possibility before going down rabbit holes unecessarily.
So in my best bodge we have stuck a piece of plastic sheet over the battery and stuck it down while Kate finds a new rad. If it doesn't happen again while that sheet is in place and after the rad leak is fixed then I think we can be fairly sure that was the cause.
If it does happen look at the battery light on the dash and pull over as soon as you can to check the moisture around the battery.