First I took the thermostat out and checked it in boiling water. Unfortunately I don't really know how much it should open, but I wasn't impressed, it didn't look to me as though it moved. So decided to leave it out for now, which is how the previous owner had her for years. Very carefully filled her up with all possible holes open, and massaged all the pipes until I was as certain as I could be there were no air locks. Checked in a tin of K-Seal (same as RadWeld) to try and block any holes, and then put the 20psi cap back on.
Did a very scenic long-winded run yesterday (25 miles) and a return trip to the Newark Auto Jumble today (75 miles). At no point did the temperature gauge leave the white box at the bottom of Normal, even when I was held up in a local village where road works have meant a 4-way set of traffic lights (which I arrived at at the wrong time in both directions) Played it very safe and switched the fan on the moment I stopped, so it maybe wasn't a proper test. But not interested in experimenting yet. I guess you could argue I shouldn't be running her that cool, but I was still kept cosily warm by the air coming up through the footwell, and the bonnet was nice and hot when I stopped so I am happy with that. I think we also have to remember that in this case the temperature of the engine core may not be the same as the temperate sensor at the thermostat, as that is one of the first points that is exposed to air when the coolant level drops and so will give incorrect values.
So quietly confident (again ), but only time will tell. I now have to get out of the habit of looking at the temperature gauge every 5 seconds.