Don, I think only certain seeds need to be put into an airing cupboard or heated propagator. Once their little heads pop up you need to bring them out of the cupboard.
If you can only grow them on the windows sills, cut an *L* shaped piece of card to fit your seed tray (one side slips under seed tray the other is like a mirror standing up the back) and cover it with foil to reflect the light. Perhaps turn your seedling every day.
Lauren :)