It's always a gamble to sow tomatoes early. This year, being colder than normal, means that the gambol has not paid off. Other years might be fine
Tomatoes must have high levels of light, otherwise they will grow spindly and weak. What you will be best doing is put them in the greenhouse each day once it has warmed up above 10c, then moving them back indoors during the nights. It's a pain, but not too bad if you get organised with shuttle trays.
Keep checking your max/min thermometer in the greenhouse to see what night-time lows you are getting. Older plants seem to be more tolerant of the odd cold night, but younger ones seem to suffer below about 7 to 8c so wait till temps stay above that.
Plants sown later in more favorable temperatures and light levels often overtake the early-sown, and are far less work since you don't have to keep moving them around twice a day.
