forecasting a frost tonight here in the midlands , I cover anything tender in the greenhouse with a double layer of bubble plastic but my greenhouse is 1/2 block walls and double glazed

they will need some protection on the night. When they are about 4 leaves or larger they will be ok overnight unless a very severe frost is forecast.
my seedlings are in a propagator at 10-12 degrees on a night. but uncovered during the day. hope that helps.

Thanks for your replies. I do have one of those plastic greenhouses inside my glass greenhouse ( for extra shelving) so if I zip that up at night perhaps that would be ok?

Hi Pdh :) My GH isnt heated but has Bubble Wrap Insulation,My pricked out seedlings r in there & i also cover them over with Bubble Wrap in the eveings just in case it gets 2 cold & i remove it if it gets warm in the day time