ECU removed from the equation (and the car). The light remained on when the ecu was removed and there was no water to be found.
It looks as though it's a chafing issue at the point the loom goes through the firewall. If I push or pull the loom at that point either side of the firewall I can make the light go on and off.
One of the Emog guys had a similar,though more immediately disabling, issue with an M16 Plus 4 about the time of Mog 2002 in Gaydon, I suppose it is something that will have affected others too.
I am not sure where to go next. Inside it's behind the heater and in the engine compartments it's under the air filter and not exactly easy to get to. I guess I have to take the air filter out and take the loom apart at that point so I can repair the affected wire(s). There is little or no free play to work with possibly as the loom must split just before that to get to the instruments.
If anyone has an easier approach.....
