Do you have a max-min thermometer in your greenhouse? I would suggest you use one as a guide, tomatoes do not like temperatures much below 10 degC in the evening. Also check the soil temperature in your greenhouse beds…ideally this should around 10-12 degC. If you can keep your young plants in good light and warm then I would wait until mid- May before planting out. Some growers I know would not plant out until the first flower truss had appeared…they reckoned that by waiting until the first truss had formed the plant then puts most of it’s energy into fruit production rather than vegetative growth. May I suggest you do a few trials and perhaps plant out some( same variety) plants now, hold some back and plant later and see what happens. I am planting some of my plants in straw bales this year for the first time in the hope that the extra warmth from the straw may help somewhat!
I,like Charles, have had a good crop in spite of low evening temperatures but if you are only growing a few plants then I would try to maximise your yield by holding on until evening temperatures stabilise. Also don’t forget that light levels are often more important than temperature.
Alan