jane writes

Just put a tiny parasene paraffin heater in the mini greenhouse, too.
That's obviously not got a thermostat on, but seems to keep the temp
to about 8C in there as against the zero it is outside.

That sounds impressive. Is this 8 deg C during the day, or do you mean
it doesn't drop below 8 deg even during the night?

How much does that cost to run?

I have off-peak electric heating in the greenhouse, which is double
glazed and has additional bubble wrap in the winter. I keep it at a
night time min of 40deg F as a compromise between what I'd really like
and size of electricity bill.
--
Kay