Sowing Courgettes Outdoors?

I sowed a few in 1st week March and some last week March in pots. I potted on into 3.5" pots on 25th March and Yesterday. I've left 5 plants uncovered in a cold greenhouse and we have had several frosts. They are a bit behind the ones in our west facing conservatory but not showing any signs of damage or stress. I've quite few squash/pumpkins germinated now and reckon, when I pot them on and they need more space, that I can leave them in the greenhouse and may be cover with fleece when frosts are forecast till it warms up enough to plant out.