I use a paraffin heater - does work a treat if tricky to get the wick size right, not too big to produce lots of black soot or too small that it burns out.
Solar lights are cheap, a shed one costs about £10 on amazon. Works even in winter.
The cloth is a good idea as the glass gets dirty all the time and it is suprising what a difference a clean pane makes in spring.
If you have anything tender then collect bubble wrap to place inside the greenhouse to keep the temperature level over the worst of winter. Remove in spring. I reuse the stuff found in packaging, though am sure the bubble wrap sold in shops might be different.
You could also try a greenhouse in a greenhouse - those cheap £10 plastic greenhouses (cheap staging!). That should give an additional 1-2C difference to plants in there in winter and is the difference to freezing and not!