Nicholas, you only learn by asking and we all started at the bottom. Make sure your grow-house is in a sheltered spot preferably facing west and out of prevailing winds, wind chill is the killer as well as too much heat.
Leave it closed on cold or cool days and open if warm for air to circulate, if you are at work all day then open it up in the morning and close at night.
We are all as Monty Don said tonight at least twenty days behind and we are all sowing seed late, it will come later and be there when it would normally have gone over so we should still be eating tomato's in October, that is quite regular here in the North East of England.
Leave your onions until the soil warms a bit then put them in the ground, the less disturbance they get the better.