While reconnecting all the engine stuff, I noticed that the ECU water temp sensor wire is hanging on with barely a couple of threads:
It's a specialist connector, not something you can get at Polevolt or the like. And that's breaking too close to the connector to easily solder a new bit of wire on.
Looks like it's off to ebay to buy either a complete Rover engine harness, from which I can rob the right connector and splice it in, or there's one guy who will sell individual connectors - just waiting to see if the harness he's getting them from is the correct one or not.
If all else fails, I can use a couple of 2mm female spade connectors, which seem to fit reasonably well into the other side of the connector, but it's a nasty bodge of a solution.