The final top 2" layer of sand was then completed:
and depth checked:
Finally, this is it with 8 seed trays in position. Each one is effectively a propagator in it's own right so that it's lid can be removed when seeds show signs of germination, without affecting any of the other trays. To the right you can see the thermostat - this is a fancy one I already had and the probe is on a cable so can be placed inside a seed tray rather than in the sand if I wish. However, it is really an overkill and the standard Parasene thermostat would be fine:
I've set the thermostat to 25C as a test tonight and will check in the morning. I filled one seed tray with compost and put my max-min thermometer on top before covering with the lid. That should give me a good idea of how warm the compost in the trays will actually reach when the sand is at 25C. Fingers crossed!