The L diesel uses the PRT (Pressure Relief Thermostat) style stat, which is a sealed unit with a bypass.
There are several different types used across the rover K,L and V8 engines.
Cream opens 87°C with Medium Spring: Range Rover and K series
Black opens 82°C with Hard Spring: L series/Derv
Grey opens 82°C with Light Spring: K series/MGTF
The harder spring rate stops it opening at low pressure, at low RPM.
The Black one is about £30 from Rover, you can pick up a Cream one from Landrover for half that. I think the grey one was part of the fix to stop K series HGF and isn't really suitable for the derv.
Info courtesy of Null-byte from Rovertech. (http://www.rovertech.net)
Only applicable to the earlier L series which doesn't have the combined thermostat and oil cooler unit.