Zee clue is in Clee's post....
Quick and dirty, don't pass go and collect 200 quid...
Go down to Maplin Electronics, you will find one in all major towns, go to their electronic counter ask for a 1/4 or 1/2 Watt 390 ohm or 370 ohm resistor and give the blokey 30p or what ever it is...
Get back to car, fold the lead wires back on the resistor so it stays put and push into the connector, with zee old sensor disconnected init (car side of the loom
)
Fire the car up and observe... drive around like a loony for a while, and do your hot tests, if that fails you will be shortly passing lots of dosh on to someone else for an exchange ecu, chipping options available
The resistor is simply fooling the ecu into thinking the water temp is around 80 degC. This is safe and won't cause you any problems for a reasonable test period; it's not a permanent fix. Note, if you test this on a particularly cold day the car may be a little reluctant to start, as you won't have the cold running additional fuelling. However, I'm sure you know what you are doing and will be able to spot this, but it should start without issue. If you want to be extravagant, and apart from joining the club benefits for you to decide... you could get a couple of values, say a 320 ohm as well, and that will simulate a cold start.
Enjoy!