What would you do

That's a long sentence (doesn't have one full stop even). Until you mentioned work near the end, I was going to say just wait a few more weeks till it warms up, when judicious use of an electric heater over-night might be doable. Do you need to work in the greenhouse? I've been known to sow seeds on the kitchen table. If your greenhouse is large, you could put up a temporary (or permanent) partition so less to warm up and may be insulate?