Is the greenhouse in a sunny location? What type of flooring?

Just below freezing at night, or 24 hours? How many degrees above ambient temperature is it? What kind of glazing?

It has been below freezing here for 2 days day and night.
I have found some bubble wrap and intend to wrap it around the pots.
Greenhouse gets the sun from morning till 2 til 3pm, sunniest spot in the garden. Floor is mostly slabbed with earth bed running the length of the greenhouse, this is currently covered with gravel.

Sitting at 0 degrees at the moment minus 4 outside

Scotrose given that a lot of the UK has had temperatures below freezing day and night for at least two days now it is hardly surprising. What you are reading though, I assume, is the air temperature in the greenhouse, not the soil temperature. If your plants are in pots then make sure the pots, not the plants, are wrapped in bubble wrap or similar; or get hold of cardboard boxes, larger than the pots, part fill with shredded paper or similar and put your plants into these. You are not going to be able to keep the air temp. above freezing easily.

You have not said what plants you have in the greenhouse. We grow just a few very tender plants and these have been moved to the, well lit, front porch with a frost-stat heater. The tender cyclamen are on the window cill of the spare bedroom which is only lightly heated. Other plants are still in the greenhouse. Our night-time temps have fallen to about -4 rising to zero during the day. However, the glass in the greenhouse give some solar gain so the overall temp is not too serious. Some plants are in lightweight plastic pots, unprotected, which I regulary squeeze gently and these have not frozen. I would not waste electricity trying to warm a greenhouse.

Bulba is right. Even with a small greenhouse like that, it would take more than a thousand watts of heating cable to keep it above freezing under those conditions. Usually not worthwhile unless you are trying for commercial quantities of early cool season crops. Wrapping the greenhouse with plastic film with spacers to keep it from touching the glazing, or with bubble wrap, can add a few degrees. If better weather comes, or in early spring, you can increase the night temperature by keeping 20 liter water cooler bottles filled with water dyed with dark food coloring. They will absorb light during the day, turn it to heat, and slowly release it at night . I would probably keep about 4 in a 6x8 greenhouse.

Wrapping the greenhouse would screen out more sunlight - the sole source of heat.
The greenhouses I've seen have hot water copper plumbing under the floor.

It is something of a trade off. That's why one layer of wrapping is better than two, and straight clear plastic film is better than bubble wrap. Double glazing is best of all, but expensive.
Copper or PEX hot water tubing is the most efficient means of heating for large greenhouses, especially in colder climates than the UK, like yours, Bathgate. It means hiring a skilled installer, and a source of hot water. If Scotsrose wants to make the investment, that is the best hope of year-round gardening in Scotland.
The Arboretum at Flagstaff, in northern Arizona, maintains a passive solar greenhouse above 2100 m (7000 ft.) elevation. While it gets colder in Flagstaff than in Scotland, there is a lot more sunlight available in the winter than Scotland, too.

Hi, welcome to GoY, there is a formula for working out what size of heater you need to heat you greenhouse to whatever temperature, but I can't remember it at the moment, I know you need the size, material, lowest expected outside temperature, whether you have a glass to floor or on a brick base etc, but for now I would get a 2kw greenhouse fan heater, with a frost setting which will keep that size of greenhouse frost free, especially if you use a plastic liner as above, I have this type of heater as back up for my main heater, which is a 6kw propane heater, [just in case I run out of gas during the night, it will be frost free].
You would probably find the formula if you google it, but it's years since I last used it, but as I said that size of heater should be adequate, Derek.

well the thermometer is reading -5c this morning and even though the sun shone yesterday it didn't get higher than -2c in E Yorkshire.
I gave up heating the greenhouse years ago as it was cheaper to buy replacement plants than heat a 20x10ft.
having said that most plants I have in there are kept on the dry side and I don't lose many now.
welcome to GoY too from me.

Thank you all so much for your help and advice. Plants pots wrapped in bubble wrap hopefully that will do the trick.
I am a new greenhouse owner and appreciate the friendly responses. Will definitely be a regular on this site.