As far as the old steam driven thermo's go, I have one in there also and used it inside and out to calibrate and double check. The greenhouse is a 6x4 which has twin wall polycarbonate but no further insulation as I have nothing in there yet. I have moved the outside sensor several times and it is consistently warmer outside at night. (Obviousley it is warmer inside during the day) I suppose I will just have to use the heater when I plant up or call in ghostbusters.
Cheers all
Darren