Probably fluctuating temperature - if it gets too cold, plants can develop whitish patches Your greenhouse probably gets hot during the day if the sun's out, and cools off rapidly overnight.

Ok, thanks. That sounds quite likely as it has been very cold at night.Will they recover, do you think?

Hopefully they'll be fine, as long as they don't continue to suffer in this way! Might be safer to bring them inside the house at night - it's not just cold nights, it sunny days heating the greenhouse up quite a bit, so the contrast with night time temperatures is the problem.

Thank you Bamboo - and for such an instant response too!

Yea, well, I'm supposed to be working, lol!

I cover everything with bubble plastic or an old bit of twinwall polycarbonate left over from a job we did and it does seem to help but the toms and peppers are still in the propagator with the lid off during the day

give it a feed of tomato feed once or twice a week , it might get a bit of ooomph back into it

Not sure about the tomato feed? It may be a bit early for that but a general all purpose feed won't do any harm. The most important thing is to keep the temp. regulated. Open the greenhouse door during the day and if temps. are going to fall a lot over night then either bring them in (As Bamboo says) or light the heater or wrap in fleece!

Thanks everyone. I covered my babies in fleece last night as I don't have a heater & it's too much of a trek to bring them back into the house - the greenhouse is next door but two! I don't think the problem is food: they are in good soil & can't possibly have used up all the nutrients yet.
I love gardening but it's all such a worry!!

Try big plastic jugs of water near the plants. They will soak up the heat during the day, and release it at night.