Went out for a long (40+ mile) run to check it out. As an aside, the weather turned while I was out so for the second time I had to put the top up.
Actually I was pleasantly surprised. I still had lots of headroom. The windscreen wipers worked well and even the windscreen demister worked as advertised. On the downside I still haven't figured out to elegantly get in and out of the car with the top up. But I don't think any other Robin Hood owners have figured it out either. The other problem was the hour it took to get the car cleaned up. However, from now on I won't let rain stop me from going out.
Anyway, during this trip I found the car overheating again and when I got back in the garage the 'kettle' was boiling again and after a few minutes there was quite a large puddle of water on the floor.
The location of the puddle made it almost certain that it was the expansion bottle overflowing. So after some thought I came up with:
(1) Maybe the expansion bottle radiator cap was faulty, or weak, thereby allowing the water to drain too early.
(2) I might have a leak somewhere underneath the expansion bottle.
(3) Maybe I was overfilling the system and it was simply venting the excess.
So for (1) it was back onto E-Bay for a new cap, again this time I got one from a classic Mini. I was also aware that you can get various pressure rated caps so I went for a fairly high one (14psi).
For (2), I added an extension pipe to the overflow and moved it right across to the other side of the engine so I could tell if the water was actually a leak or a genuine overflow.
Then it was off for another run. Interestingly it very quickly reached a very high working temperature, although not in the red, and for the rest of the seemed to sit there in the same place. And reassuringly, when I was forced to stop at traffic lights the fan came on and it maintained the same position. So my new thermostat switch is working well.
Once back into the garage there was no sign of the 'kettle' boiling and there was only a small puddle of water, definitely under the overflow pipe and not from a leak. When I took the bonnet off the engine and water system were hot but nowhere near as hot as I expected.
So from that I deduce the radiator cap is now working, there is no leak and the system will now maintain temperature, although a bit higher than Normal (maybe the temperature gauge is overreading?). Also, now I know that it is the expansion bottle overflow that is generating the puddle, maybe I have been overfilling it.
I have a couple of long runs planned for the near future so will see how she behaves. No passengers so will have a couple of litres of water stored in the footwell just in case !!