As this system was perfectly usable I left it alone initially, filling it up every couple of weeks. It is rather an odd design in that you fill it up via the expansion bottle. However, on 3 occasions I noticed water coming from underneath after I returned from a trip and decided it must have a leak somewhere. But further investigation showed the water was coming from the overflow pipe from the expansion bottle. To make sure I installed an ‘expansion’ bottle to the ‘expansion’ bottle and ran it with the overflow tube into the new bottle. Not one of my better plans as after about 15 minutes I saw water cascading into the passenger footwell. Turned out the new expansion bottle had quickly filled and overflowed as well.
It was after I had refilled it from this incident that I found part of the problem. I had done up the pressure cap on the expansion bottle to what felt like full tight, but on this occasion I decided to make sure and by applying more pressure I managed another quarter turn. Since then no water leak.
A test of the original thermostat showed it was probably seized, so a new one was purchased off E-Bay.
I then found that shorting out the thermostatic switch made the fan run, so decided the switch was broken. The general advice on the forum was not to try and replace it, as apparently it was a poor design (not screwed in, but a bayonet fitting) and incredibly difficult to seal a new one. So I decided to buy an after-market adjustable thermostat switch and fit that.
Status: Thermostat and switch awaiting fitment.