I am a great fan of Epsom salts as some of my plants get magnesium deficency and I thought this advise from the National Gardening ASS would be helpful to you.
'Magnesium deficiency in the soil may be one reason your tomato leaves yellow between the leaf veins late in the season and fruit production slows down. Test your soil every 3 years or so to check on nutrient levels. Epsom salts can keep plants greener and bushier, enhance production of healthier fruit later in the season, and potentially help reduce blossom-end rot.
Apply 1 tablespoon of granules around each transplant, or spray a solution of 1 tablespoon Epsom salts per gallon of water at transplanting, first flowering, and fruit set.'
I am not a fan of spraying it on and I tend to apply mine dry around each plant and work it in to the soil with a light hoe and if it has not rained in 48 hours then I water it in. Great for tomatoes Clematis and roses. Ps you can now buy it in Garden centres etc for under £4 for 1.5 Kgs so it has come down in price.