Thanks for that diagram. Sorry I didn't finish my dse diy electrians kits so doesn't mean too much to me so I'll see if my sparky or supplier of thermostats can confirm how it'd work with what I've got. The heattrans had 3 speed controller which might put a spanner in the works or fit half the puzzle...
Then give 'Rubustat' a google. You will see a Github repo at the top. It will be a very good start as that is the main bulk of the programming you will need. From that you can screw with it as you please.
chevrolux: Raspberry Pi + bunch of DS18B20 + relay pack.
Looks like a lot of fun but I can see it being winter before I can get all the parts for this along with finding the time to tweak it.. Also as this is a 240V system it could cause a problem with insurance / cert of compliances etc..
What can be simpler than two thermostats (you may already have in some form) and two relays? Sure you can program a Rasberry Pi or Arduino, or a PLC, or a microcontroller, depending on your skills. But you can buy relays at Corys and your sparky can wire it.