Well, I cannot take credit for this idea, that goes to Bilbowaggins on her 'View Fom Bag End' blog.
Her propagator is situated inside a mini greenhouse, within her own large greenhouse. On the coldest of nights she lights night lights to keep the frost at bay. What a brilliant idea for those of us with no room to keep all our seedling and young plants on a warm window cill. The plants are also getting plenty of light, so should not get so leggy.
It is also more convenient to open up the mini greenhouse for air circulation and extra light during the day.
Thanks again to Bilbo for letting me share this great idea. Which I am sure quite a few other people probably do. However, seeing most of my growing experience has come via commercial systems, where we always had the equipment to raise plants in near perfect conditions, I am constantly learning ideas that everyday growers have adapted to suit their own pocket and conditions.
The wonderful thing about growing, is that daily you tend to learn something new.