Leaves on Tomatoes going Transparent

Hi,
The leaves on some of my Tomatoes (mainly on the lower leaves) have goen transparent in little patches. This isn't on all the plants just a few. Some of the other plants have leaves (again on the lower ones) have gone pale, thin and have curled up.
They have been in a greenhouse (wilko one with a plastic cover). I have kept it zipped up most of the time as it's been so cold and am wondering if all the condensation in their is to blame??
I have taken them all out and left them on the patio for today in the hope a bit of fresh air and circulation will do them some good as itr is 14 degrees today. Will put them all back in this evening. Also left the door of the greenhouse open to get some fresh air & dry it a bit.
Any suggestions as to what's caused this would be good. Thanks

Mine too in the last few days have developed "thin patches". No other sign of distress. This coincides with the fact that I have started putting them out for a few hours during the day, so I was wondering if it could be cold damage or maybe sun scald...

Thank you people - sounds like all is not lost. Epson salts and I think the greehouse is dry now. I normally do water with rain water from the bottom, but have a feeling i left this to Hubby the other night whilst I was at work and he's probably given them a good shower as well