Tomato Success Kit
It sounds as though your plant may be suffering from a magnesium deficiency.
The solution to this is to apply Epsom salts as a foliar feed. You mix it up using 20g of Epsom salts per litre of water. Apply this to the foliage via a spray in dull weather to avoid scorching the leaves. This needs to be carried out 2 to 3 times a fortnight. I would also pick off the yellow leaves currently on the plant.
I hope this helps and that your tomatoes thrive, however if you need any further help, please ask.