Probably magnesium deficiency. This frequently shows in plants as a browning of the foliage after first turning yellow. You could water the plants with dilute epsom salts water (8oz per 2½ gal). Apparently, over usage of high potassium fertiliser starts working against the plant and makes it harder to absorb magnesium. A bit of a catch 22 situation here as tomato food is rich in potassium!

Thanks for the suggestion. After a search on the internet, that was my first thougt. I have tried epsom salts a couple of times, but more dilute than you suggest. Maybe I will try again with a stronger solution and pass on the feeding for a week or so and see if that helps.

I have had pounds of tomatoes off the plants so far, so don't want to do anything to stop them thriving. However the leaves are slowly all going like this so they won't be photosynthesing too efficiently and that will slow down the tomatoes, so I have to try something. Whenever you see plants on Gardeners World or in the magazines they always look so green and healthy, but my experience of veg growing is unfortunately somewhat different, despite working hard to get it right!