I understand that seeds should be started off in the dark warm place
and the seed trays should be watered & covered with polythene.

Seeds vary considerably in their germination requirements. For instance,
sunflowers don't need a dark warm place, grow tall very fast and you
won't be able to put them outside for at least a couple of months yet.
It's too early to sow those.

Read and follow the instructions on each packet, don't treat them
all the same.

Janet.
Thanks for your reply Janet.

the packet of sunflower seeds says. "store in a warm place, an airing
cupboard is ideal"

It does say if your planting them outdoors do it in march - june

I have a greenhouse, with a little electric fan heater in it, but were
new to gardening so havent really used it.

But none of the packets say what soil to put the seeds into.

Thank you for your help.