At the MOT the mechanic had noted a small weep at the thermostat housing, and I was finding more and more water on the floor of the garage. So it was time to change the gasket. I had bought a new thermostat a few months ago and it come with it's own 'generic' gaskets made of paper. But when I checked on E-Bay I found it should be fitted with a rubber gasket.
There were actually 5 for sale so I bought 2 so I have a spare (I did wonder about buying all 5 and becoming the sole supplier)
So whipped off the thermostat housing, saw the paper gasket has actually split, popped in the new rubber one and put it all back together.
I filled up the system and went off for lunch. When I came back there was water all over the floor. I hunted high and low for the leak but had no success, but I could see the thermostat housing was still weeping a bit. So this time I took it all apart, cleaned everything, removed all trace of the old gasket and made sure the 2 mating surfaces were perfectly clean. Put it back together and there is no apparent leak and after a long drive the system was still full and after a few hours there was no water on the garage floor.
I also took the advice of a fellow Superspec owner and replaced the 13psi radiator cap with a 20psi cap to stop the system overflowing.
And while I was checking it all I took all 4 spark plugs out to see if one of them was different. In fact they were all a nice chocolate colour, meaning the mixture is correct and that a gasket problem is actually unlikely.