don't attempt this unless you know what you are doing - this guide is intended for those who have "done it before"

A less than enthusiastic heater led me to suspect the thermostat and sure enough, checking on the OBD port confirmed that the hoped-for 83°C was only a reluctant 69°C even in the very mild November 2011 temperatures.  Measuring the temperatures of the actual air flow (fans full on) showed similar discrepancies.

However when re-checked at an air temperature of +3°C in March 2013 the max temperature read by the OBD only reached 64°C  - so is the new thermostat (just over a year old) or the temperature sensor suspect?  My wife's car was still showing 84°C at the OBD so nothing to do with the ambient temperature.

The thermostat itself is quite complex as it incorporates the water temperature sensor and two pipe stubs, as well as the thermostat itself all in one unit.  Removing the thermostat is not straightforward as the coolant has to be drained and several water pipes disconnected.

Start by removing the air filter assembly and then its support bracket (2 bolts); also the bolt that holds the support for the adjacent water pipe.

The pipes are held by very strong clips - use a mole wrench to pinch the ends to slide them off and replace them by conventional jubilee clips on reassembly. 

It can be helpful to disconnect the dip stick tube and the bracket and the connector to the coil pack to move the wiring around to get access to the thermostat - though not essential.

Remove the two rubber hoses (the heater hose is hidden so can only be disconnected after the thermostat is out), the temperature sensor connector and the four bolts holding the thermostat.  I didn't drain the coolant first and instead let it run out as I pulled the thermostat out (afterwards washing the area down with clean water).

Reassembly is the reverse of dismantling, as they say.

Clean the mating surfaces of the thermostat housing - I didn't use any sealant.

Firstly slide the heater pipe and clip (don't tighten yet) onto the thermostat housing.

Carefully position the thermostat and its integral gasket and insert the four securing bolts; tighten down evenly.

Bolt on the air cleaner support bracket (two bolts) and the water pipe support bracket.  Tighten the heater pipe clip and refit the larger hose to the thermostat; tighten the hose clip.

Refill the water reservoir - I used this device made up from an empty drinks bottle and old reservoir cap as you will need a head of water above the reservoir to fill the heater matrix.

There are two small black plastic bleed caps - on one of the heater pipes near the bulkhead and on the thermostat itself.

Adjust the heater to supply maximum heat to ensure the heater valve is open, but keep the fan very low or the engine will never warm up!

Fill the reservoir, start the engine and remove the bleed caps one at a time. Close the caps when there are no more air bubbles coming out. Keep filling the filler until the reservoir is up to the full mark and allow the engine to run up to temperature; recheck the reservoir level and that there are no leaks.