When you say "half way up" you mean between the min and max marks?
The proper level is lower than you'd expect (apologies if you know this, just checking).
I understand there is a type of failure on the 1.8 where it leaks coolant into the engine (or out) via the inlet manifold gasket - but that won't apply if you're not losing any water.
I would try running it (stationary) using that IPK temp monitor and see what temps you're getting. Somewhere on here is listed the fan cut in temps etc.
That might give a better idea, although if there's nothing obvious, it may be time for a trip back to the garage that did the thermostat etc.
There's a process for ensuring the system's not got airlocks after a refill detailed by another member
here