It turned out that the thermostat housing itself was cracked and though it was only losing a few drops at a time, that was enough to empty the cooling system in a very short time. I bought a brand new replacement part from eBay and spent Saturday morning installing it. I had to smash the old one with a hammer to get at the valve inside and in doing so discovered that underneath the oil and dirt it was made out of green Bakelite.
This morning we found a really nice boot sale in Chorley wood where I bought a pair of unused 35 year old BMC seat belts and a huge chrome spot light for her. Toby and I met a guy who has a music promotion company, is an avid vinyl collector , plays keyboards and lives round the corner from Toby too. i had another of those days where everyone wants to stop and have a chat about Althea and was quite happy to do so.
I slept in her last night and once again got a lovely night's sleep. It got really cold in the night but as long as I kept all the vents shut, blocked off the gap under the door, stayed under the duvet I was warm. I'm looking forward to the winter now and experimenting with how cold/warm she is.
Here's a picture of Boris the spider. He's been living in Althea's cab for about 5 weeks now.
And here's a picture of Plum at the boot sale.