When fitting the 1.8 diesel thermostat I installed it with the rubber gasket that came with it. Unfortunately this is slighly thinner than the Zetec petrol thetmostat gasket, and as such I had a small coolant leak from the thermostat housing.
I've taken it all apart again and fitted the original petrol thermostat gasket to the diesel thermostat. Just been for a test run and no more leak. Coolant temperature still fixed steady at 88 degrees. Finally happy.