Poor tomatoes

It's Blossom End Rot, it's a lack of calcium in the developing fruit - most people think that it is lack of regular watering, you may water every evening, but if it's not enough, they may run dry before the next watering.

Some people like to add a little garden lime or other source of calcium to the soil before planting potatoes, just in case their soil is deficient.

Site info here,
Tomato Blossom End Rot | Causes & Cures

I think that rather than "little and often" as a watering technique, you should give a proper soaking once a day. I give each grobag 10 litres each day and feed with Tomorite once a week. As you can see on the photos I've just taken this morning, mine are looking OK on it!

I love the chair next to them Pesky, is that so that you can gobble them as soon as they ripen? 😂😂😂