Posted: 15/05/2017 at 18:55
If you're growing them that way, they need loads of food and water, and properly tied in as they grow, but they would have benefited from being pinched out a while ago to make them bushier. They may also have been planted out a bit too soon, and could be suffering from damage by wind. If you bought them as plants at that height, they were probably grown from an autumn sowing and in very protected conditions. They may well not have had a good enough root system to support the amount of top growth.
You're better feeding the soil well - loads of well rotted manure, especially if your soil is light and free draining. A tomato feed is good, but shouldn't be necessary that early if they're in the right soil. Full sun isn't that good for them either, especially if you get high temps. They don't like that at all.