Jonny, if you could get your hands on an Oscilloscope you could scope the primary winding of the ignition coil. This will tell you alot about whats going on. A large induced voltage shown on the screen will confirm that your getting good compression and that the coil is doing its job (the higher the compression the more voltage thats induced). also from this you can see the spark duration, switched earth from the ecu and main feed for the coil. I dont think that it will be the main feed for the coil as they all share the same feed.
All this aside, a coolant temp sensor will make the car turn over more on crank and on some cases cause a missfire. Check out the voltage going to this (should be 5v) at the plug (unplugged from sensor) and plugged in with ambient temp around 20'c' a voltage of around 3.6v.
Hope you get to the bottom of it.
Iain