Lack of calcium in the fruits caused by irregular watering.
They will grow through it once you sort that out.
Can also be caused if too much Potassium is present, as the plant uptakes that in preference to Calcium, but usually it's down to watering.

And which variety you grow too......

In the recent heat the toms have struggled to take up enough water to replace that lost, I,ve been spraying the plants when the greenhouse is shady as well as watering morning and evening.....wetting the gh floor can help too. Don't spray in the evening though, or now its gone cooler, the damp cold isn't good for them.

Take off the damaged ones as soon as you see it so that the plant is not wasting its energy.
I,m sure if you feed and water as usual you should still get some good tomatoes.