They could be, they could also be overwatered, young plants can't take water up very quickly especially on cold grey days.....and we hsve had plenty of those......

You're right. That rings the right bells. I water them too much. Thanks very much. I will know, next time.

Hold on,
It could also be a drainage problem or lack of it,

Ask yourself what if we have rain for days on end ???
is this not over-watering via nature!

I have lettuce and sweetcorn in now and ive had nothing but rain for days now non stop and "May" has been more rain than we've had for a long time but the lettuce and sweet corn are the normal colour,
Ive Punkins in "now" these dont like a lot of rain on the leaves so ive one of those small plastic polly tunnels over them and they're doing fine.

Check your drainage and potash feed the veg, (tomato feed will give good potash).

Thank you Dungy. I think that with the growbags that the lettuces are in it is a drainage problem. Although I have made holes in the underside of the bags, it is probably not enough and I have also been watering them. As for the sweetcorn, they are in small plastic pots. I know they don't like being moved, so maybe they didn't like being moved from the seed trays to the pots - and then over-watered! I will try some potash feed too.

One thing you could try another year for the sweetcorn are the peat pots, little discs that when you soak grow into a netted pot. When you plant out the sweetcorn you plant the whole thing so no root disturbance.

Thanks Pamg. I will try that another year.