think its irregular watering that causes it.But could be wrong
sheila

try a calcium feed
orac60

It's called blossom end rot and it is caused by irregular watering and something to do with calcium. It happens frequently if you are growing your toms in pots or growbags where they dry out and then get watered heavily again.
If you can possibly grow your toms in the soil or greenhouse border, I'd do so. I've only ever had blossom end rot when growing in pots and bags.

It happens when the plant dries out too much at night, which is when the plant is pushing nutrients into the fruit--tomatoes don't have pores in the fruit skin, so it takes root pressure to get the nutrients in. Rarely, it may happen when there is an actual lack of calcium in the soil, which is also more frequent in containers. So, I would water more often, and maybe much the soil surface first, and if that doesn't work, add a few spoonfuls of gypsum to the pots.

Thanks to you all it will be very helpfull. kind regards Marshall