It's caused by calcium deficiency, which is usually caused by the roots getting too dry between waterings. On a really hot day, they're better with a morning and evening watering, rather than one big soaking, as, If your plant’s roots get waterlogged, that may also stop the plant drawing calcium up to the fruit.
High Nitrogen fertilizer (like chicken manure) can also make the problem worse, try a proper tomato fertilizer (Poundland sell the cheap and cheerful Doff brand if you have one of those shops nearby - it may be cheap but it still works, I find)
They often just grow out of it anyway, once you've removed the first bad fruits, especially if the weather changes, so don't give up hope