So it turns out yellow leaves on your tomato plants means

[financiallyscrewed]

that your plant has a magnesium deficiency. My FIL told me this last night and it's all on the internet if only I'd thought to look there.

So, I've bought a hand spray for £1.50 and epsom salts from the chemist for £1.50 and apparently if I mix half an ounce of salt with a pint of water and spray the leaves of my tomato plant, it should be healthy within a fortnight.

Don't know if everybody else knew this, but thought it was helpful!