Hello Jools,it's hard to advise you when I don't know what you're growing and can't see what you're doing. Seedlings are much tougher than you think. About 6 degrees is a growing temperature and most things are perfectly safe around 3 degrees.
I've never used an electric propagator. I keep my planted seeds indoors until they germinate then I get them outdoors if possible , or into the greenhouse without heat. If it's cold at night, cover them up with fleece.
It's still only January, so you're quite early with flower seeds. About mid March would be fine for most things. I plant my dahlias then and they flower beginning of July.
Today I rescued some lupins. I planted them outside in seed trays late last summer. For reasons not to be talked about they were ignored until today. I found them in sodden seed trays which had been out all winter, lovely little leaves on them and none the worse looking for the neglect. I planted them all into 5" pots and left them outside. (garden centre charges 99p for them) and I'll plant them out later and expect them to flower this year.
Heating the greenhouse costs money, and I try not to heat mine til about end of March, through April when required and frosty nights in May, and to have enough in there to justify the expense.
If you tell us what you are growing, we can give you more specific advice.

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