Tomato Seedlings

Hi all,

Need some advice (as always).  I have 24 tomato seedlings which have now been potted on into 3 inch pots and popped back into the heated propagator.  They look to be doing really nicely (1 or 2 are a little leggy but not too bad all things considered).   

The weather is still very cold here.  I am just wondering as they are now starting to put out their second sets of true leaves if I should lower the temp in the prop (currently at 18 degrees) or take the lid off the prop during the day to let them get some air (which will naturally reduce the daytime temp) and then just put the lid back on at night.  The prop has an extension which allows me to increase the height so there is no problem with them staying in there for a good while longer.

If needs be I have a heater for the greenhouse and could take them out altogether.

I would very much appreciate your wise guidence