I do have a heated propagator so I shall use it to germinate the seeds, then gradually wean them to the ambient temperature of my small GH. by bringing the germinated seedlings into the house for a short time, then back into the GH again when they are more sturdy.
The GH temperature has never gone below 5C at night, it being a lean-to type against the south wall of the house with double wall polycarbonate glazing. Bear in mind too that I am in the south-west where we seldom get what I call a frost - only minus 2 or 3C if there is a really cold spell. (I have had a pond for five years and it has not iced over yet!)
Tricia