[bigandquick]

You'll need a torx bit to remove the two bolts on top of the themostat housing. To minimise coolant loss I undo the thermostat to radiator hose only.
To ease access to the bolts I release the bracket securing the fuel pipes and just move them over slightly. The ECT sensor sits under the thermostat and simply screws in.
There has been recent discussion regarding the manufacturers of these and a compatability/reliability issue. For that reason I'd recommend biting the bullet and for a couple of quid more getting an original one from a dealer.