Ian, if like me, you want to avoid batteries and electronics in the greenhouse, there are two main 'old fashioned' types: those which appear to have two mercury tubes, a max and a min, and those using a bimetal strip. The latter will almost always be in dial form, and although generally cheaper, become unreliable quite soon because they have moving mechanical parts. The cheapest mercury tube type I've seen was about 7 quid but they last for - well, I've had mine for five years.
Geoff.