Jupiter writes:

This must be ever so common this year. I've got about 30 outdoor
plants similarly affected. They seem to have acquired the problem
when in cold frames. I noticed small tomato plants for sale at my
local market today with the same signs. I'm going to try a fungicide
(Dithane 945). I don't think it's any kind of nutrient deficiency
causing it.

Interesting. Nice to know I'm not the only one (well, you know what I
mean!). Keep us posted as to whether the fungicide seems to do anything. I
wasn't going to spray anything on mine, but maybe it's worth a try. It'd
be a shame to loose the plants! I think I'll see how they look after
they've had a few days of warmer/dryer whether.