Heather, still not easy to see. Seems the site is having problems dealing with images. Anyway, it's a closer look and, yes, it could be a magnesium deficiency.
Have you been feeding the plant? Tomato fertiliser is usually high in potassium which can interfere with the plant absorbing magnesium. If you have been feeding it, stop for a while. Good old Epsom Salts is the best source of a dose of magnesium. Before you water next time, sprinkle a couple of tsps of ES onto the soil, scratch it in gently, then water well. If things improve, that was the problem. And I'd leave off fertilising it for a while.