The symptoms of blossom end rot are sunken patches at the blossom end of the developing tomato. These patches develop hard skin and turn black. The condition results from dryness at the roots which prevents the roots from taking up calcium. The most common reason for this is erratic watering. Don’t worry – just because some of the tomatoes are affected it doesn’t mean they all will be. Pick off all the affected fruits and make sure you don’t allow the compost/grow bag to dry out. The yellow leaves may be symptoms of magnesium deficiency – this can actually be exacerbated by high levels of potassium contained in tomato feeds. Use J Arthur Bower's Epsom Salts to provide magnesium. The quickest way is to make up a solution using 70 grams in 9 litres of water – use weekly.