The secret is a fairly stable temperature with moist, but not waterlogged compost. These seedlings will be pricked out into individual 2" pots to start with and will go straight into a heated greenhouse with a constant temp of only 10 degrees C. This again keeps the growth slow, giving compact stocky plants that will go out into the tunnels sometime in April.
The kitchen where all of my tender crops germinate is north facing so once germinated the seedlings do need to be moved into a lighter location but it also prevents any harsh sunshine burning through the window and shrivelling them up.