Buy an electric heater. Buy a heat mat. That's it.

Hi, welcome to GoY, if your greenhouse is large enough, you can divide it up by either suspending sheets of bubble wrap insulation, or with a more permanent division by constructing a framework inside the greenhouse, and glazing with either glass, polycarbonate, or clear plastic roofing sheets, and a small heater in the divided off part, Derek.

Or an even smaller area by erecting a framework covered with fleece over an area warmed by a heat mat.

My method works fine. No need for fancy framework, polycarbonate, fleece, bubble wrap. Just set thermostat on electric heater to 50'f. Thermostat on heat mat to 60'f.

or buy one of these for your heat mat

http://www.twowests.co.uk/product/cover-for-electric-blanket

I stand one of the cheap mini g'houses inside the bigger one, just lift the cover off when not needed.

I was going to say exactly the same Lincs, I use the shelving then through spring and summer....

Yes they're very useful Janey..

Last year All my Geraniums survived in the Greenhouse , and no heater . My Hubby puts large Sheets of Polystyrene ( saved from fridge etc packaging , over the years ) all around the inside of the greenhouse . Then makes a type of frame and roof , using about half of the inside area. Success .