Frost Free Greenhouse Heating?

Dear Mark

Many thanks for your enquiry regarding how to keep your greenhouse frost free during the winter months. Let's start with the good news that there are plenty of electric heaters around fitted with thermostats which will switch themselves on and off at pre-determined temperatures and therefore no interaction. 
  We stock a couple of very reliable and efficient models which I've use to good effect in our Harrod Horticultural Kitchen Garden greenhouse; in fact, I've got one set up now to combat frosty winter nights! It's the Tropic Electric Fan Heater and this model, with a built in thermostat at the rear and a designated 'frostwatch' setting, takes literally seconds to tune in and has prevented the greenhouse temperature dropping below 5°C according to the just-as-reliable max/min thermometer.  
I'd certainly recommend that the heater route is the one to take but in a greenhouse your size, you may well require 4kw of power to generate decent heat levels; this would mean using two of the 2kw rated Tropic - or similar heaters - but if you use bubble insulation to line your greenhouse or even hang a sheet of this material from the roof and section off a smaller area of the greenhouse, then one heater may well suffice.
Other options? You could try a larger propagator such as the Vitopod or Jumbo Heated Propagator instead of a heater; heat mats would generate localised basal heat but I'm very dubious they would keep a greenhouse your size frost free.
I do hope that the information I've provided proves to be of help and if you've any questions or queries concerning the products or solutions I've mentioned above, please don't hesitate to contact me.