So just to conclude on the original question - no/ bad sensor reads -20 degrees. Putting a wire straight across the connector will read up to 100 degrees, but then gets replaced by a 75 degree default value after enough time - air con works fine in this mode. Using a 22k thermistor on the connector reads you ambient temp - i didn't bother trying to clip it up to the pipe, or mount it in any meaningful way. I have a new sensor being shipped from the US for about 70 quid. Had I known then what I know now I would probably just have put the wire across the connector for the heater core sensor and forgotten about it - one less point of failure eliminated, even if it risks a blast of chilly air in the winter in the first few seconds before the V8 heats the heater core.