I have had them spot like this and recover. It really depends whether we have a wet spring. I would take the plastic of while its dry and let the air around it, but cover it if there's prolonged rain. They can get a bit 'sweaty' under plastic. I use sacking loosely around the plant, then add a plastic sheet over the top just when its wet. Also its a good idea to remove an offset and pot it up before the winter - so at least you have a replacement in the greenhouse/windowledge!

Spotting may be oedema from too much water, but mild frost damage can look similar.

Thank you for your comments. I would hate to lose the big one especially!!