We use a fan heater with a frost thermostat... works just fine

Agree. Get one designed for greenhouses.
I have had a hunt around for you and most are pretty expensive but you might like to have a look at this one

http://www.twowests.co.uk/product/2kw-greenhouse-heater

If you don't like this option there are several tubular heaters designed for the job as well

Hi, what size is your greenhouse ?, the majority of greenhouses in peoples gardens tend to be 8ft x 6ft, and a 2kw fan heater is adequate, to keep it frost free, if you want to guarantee that you will be able to keep it at 45 - 50f, depending on the type of winter we have, then youmay need a heater with a bigger wattage, you could also line it with bubble wrap, which will cut down on cost, but may cause problems with damp, Derek.