See piccie below from 3 days ago. The sungolds are starting their 2nd proper leaves after the cotyledons, having been sown on 8 Feb.
They're cheap bought domes, but a propagator with lid would obviously do just as well. I think it important to add a bit of fleece around the base.
I have a max/min thermometer in the greenhouse, and the lowest so far since the tomatoes/peppers were installed has been 0.5 degrees inside the greenhouse, not much different than from outside, but crucially protected from winds etc. Inside the domes/propagators, the temperatures stay 2 degrees higher. Another degree, maybe two if you're lucky, can be added if the plants are put on the heatsink greenhouse floor, but then there are other issues to contend with. During overcast days with, say, 7 degrees outside, the greenhouse manages anything from 12 to 16 degrees. On sunnier days, it's up to the high twenties, far more favourable than at home. The greenhouse stores a bit of daytime heat, and the lowest temperatures only last for 2 or 3 hours.
The tomatoes don't sulk or get stunted, just obviously slower growing than their later cousins. I appreciate that these type of plants don't like extremes of temperature later in the season, but tomatoes/peppers, while seedlings, seem to be more resilient to low temperatures for a few hours around 4am than some people give them credit. They don't sulk like sweet corn or commit hari kiri like cucumber. As usual, last year the early sungold plants were just as healthy and productive as the later plants, but started harvesting on 21 June, a full 23 days before the later sown seeds, well worth the extra effort. The chillies follow the same sort of time difference. I'm not quite so clued up on the sweet peppers though.