Thanks Luddite, the easiest way to access the sensors is with the S&S Protune software (available free).

Richard, no I didn't have Protune for most of the tour but did stop by at M3WServices France. Due to limited time Steve was only able to check the sensors whilst the car was stationary and still hot. There was a stored P0118 fault code (high engine temp) but nothing live. Once cleared the fault code didn't return whist Protune was connected. The ETS looked to be working OK with the temp rising from about 170f to 200f at which point the fan came on. For a number of reasons we then suspected the intake temperature sensor and changed that which seem to solve the problem. However after leaving it became clear it hadn't so the other likely faulty sensor was the ETS.

When I got home I ran it with Protune from a cold engine and the problem showed fairly quickly between about 71f (ambient) and about 140f when it started fluctuating then going down rapidly until at one point showed -4f ! On Protune the measured temp then flashed red and defaulted to a fixed 100f. So it appeared the original sensor was faulty somewhere around 140f and when we tested it at M3WServices it was probably working again.