overwintering.

What kind of things are you intending to protect? I have an unheated greenhouse with mesh doors at either end and even last year with -8c in there did not lose anything, even a supposedly tender Echeveria.
My frames have only the glass lids on, no covering at all and again we do not lose stuff. All that die for us are things which do not like winter wet, if I forget to bring them in.
However, none of my stuff is what you might call tender.

mostly cuttings that i,d taken earlier in the season, some solanum, fuchsia, cineraria, argyranthemums, osteospermum and salvia.

In the case of those plants I would think that they would all survive well in a cold frame without heat. If you get really cold weather then cover the frame with bubble plastic for the nights.
The plastic cover on a mini greenhouse is not that good at keeping cold out and the problem you may have with it, is that it will still get very hot in there during a sunny day, but drop dramatically overnight. I would only use it for the hardier plants, but needs must, so yes you could use a heater in it on the coldest nights.

cheers, will need to do it soon as the tempreture will drop to 2 deg this week so frost is not far away.

the problems with frost is when we trap it, frost flows along the ground, and it has to be able to escape your garden, for instance a garden wall without either air bricks or bricks left out near the bottom is a frost trap and that is when it dose the damage,so if you make shore that your frame or minie greenhouse can not trap frost,or be in traped frost, it will to be a very heavy frost indeed to harm your plants with insurlation, but the parifin lamp I would leave out , minde you tomatoes like the fumes,