Thank you for commenting, I amended the soil with horsehorse manure, wormcastings and fish emulsion. My other tomatoes are 4ft tall. It's just this one odd tomato that's giving me trouble.

I'd pull that one if you have other plants growing well. In the past I've spent too much wasted time on plants that recovered okay but did not produce any good tomatoes. Sorry, I'm not more help....

If that is the only plant that is doing poorly, and it is doing very poorly, I would be concerned about one of the wilts - maybe fusarium. I would remove the plant before any possible contamination can spread. Some varieties are more susceptible to fusarium than others. If you can find a branch besides the main stem and test it, split it open to see what the inside looks like.

I would also remove weak plant ASAP

What is the variety?