As the weather has turned cool after a warm period (I think that's what's been happening in the UK?) it could just be chilling. If there's no likelihood of anything like spray damage, I wouldn't worry too much as the plant will probably grow out of this as summer continues. Clearly it's nothing like blight which would turn the edges of the leaves brown, and then the stems. My other thought was overwatering if it's in a pot or a bag... check the compost isn't too soggy.

Thanks.. Will see what happens!

are you feeding them yet? they could need more minerals and a dose of tomorite mixed according to the tin wll help alot.