The earlier 1.8 engines have a conventional themostat. The later egines (from around the 2003-2004 changeover) have a Pressure Relief Thermostat (PRT). Each type is located differently. The conventional/earlier stat is in a plastic housing at the side of the cylinder head near the pump. The PRT is located in the hoses behind the bottom of the radiator.
When later 1.8s get head work in garages unfamiliar with the later PRT system, they quite often fit a new (conventional) stat in the upper housing. When this is done, the engine becomes virtually unusable due to rapid and uncontrollable overheating. It's reported on the forums surprisingly often and leads to lots of frustration and hair pulling until the upper stat is removed and replaced with the 'dummy' as specified by MGR. If you have a PRT, then this error could be what's giving you the grief.
There's an easy way to tell. Look at the large hose that comes from the RHS of the head (as you look at the engine). This turns left and goes along the top of the radiator and enters an elbow on the left top corner of the radiator. Look in the middle of this hose. There's a plastic connector. If this is a T-piece, with a large hose dropping vertically down behind the radiator, then you have a PRT.
To see the PRT you need to look under the front bumper. With the engine underpanel off (if it has one), look underneath at the radiator bottom. The vertical hose comes down and right to connect to the PRT. This is what it looks like.
Check and report back.
TC