Thanks for all your replies.
I agree, TandC, the inbuilt thermostats on the oil-filled heaters seem a tad random. I shall feel a lot more confident with the frost stats and it should save a bit of cash, too.
One other thing I was wondering about. I have an Eberspacher which runs three radiators. At the moment, it has just the mini timer fitted. I looked on the website to see what alternatives there are, but I couldn't work out which one, if any, would allow me to programme it to come on every day for, say, a week. Would this be a viable alternative or are there reasons why it shouldn't be run when the boat is unoccupied?
Kathy