yeah its blossom end rot. i myself am fighting this personaly.
i remove all fruit with the tell tale signs of that nast brown soft spot on the underside of the tomatos. good thing is BER does not infect fruit on the same plant or others in area.
i had been reading up on this. it is either a magnisium or calcuim problem. there are sprays you can use on the fruit and leaves too give the plant a boost in calcium/magn.
other growers swear on removing bad fruit and letting plant recover over a couple of weeks they normaly sort themselves out another trick i was told. use a tablespoon of epsom salts in your growing medium before planting up. BUT i have a crazy idea well not realy a crazy idea, Black treacle aka molasas has large amounts of mag and calcium as well as other nuetriants the plant could use. it also has a huge sugar reserve and would force tomato's into flowering. so i was thinking why not, the cannabis growers use it so why not try it on my tomatos.
me i went with the old way and natures own. removing bad fruit and letting plant sort itself out. mistakes i make this year i certainly wont be making next year. and i have a 450g tin of lyons black treacle (molases) ready