Wow! David! Half mine haven't germinated, did all that I should have done according to advice. They were going straggly on a windowsill in a cool room, now are in a cold frame. I used loo roll middles which are starting to fall apart. Silly me! They are bio-degradable ones!
Going back to basics, mine are never indoors at any time, they are just too hardy and react by romping away.
I just sow the seed in late October/early November in a cold greenhouse. As soon as the seed has germinated, then they go into a cold-frame.....even the lid isn't closed unless very extreme frosts are expected and there they stay until planting out time.
On reflection, I'm beginning to wonder if either sowing as above, or just sowing the seed in situ would be best.
I know this works, but if anyone feels like trying an experiment, try sowing the seed where they are to flower in the autumn. The main setback with this is protecting from slugs & snails and probably birds.
Frost & snow will never be a problem.