I am the proud new owner of a thermostat. It is one of the assembled models like the ones from Maplins.
Someone recommended asking for the thermistor (temperature sensor) to be on a longer piece of wire so that the main part of the termostat doesn't have to be inside the incubator, and this ebay seller had already thought of that so I bought this one.
The seller provided a simple image of the wiring needed to be done to wire it up in an incubator.
It cost £11 + £2.50 P&P, which is quite good.
I went to the shops today and I bought hinges, door handles and magnets to keep the door shut.