Blossom end rot caused by irregular watering. This can be because they compost is not getting wet enough but going straight through when you water(growbags are bad for this), or letting it totally dry out before watering.
It depends on the size of the pots. Smaller pots are more prone to drying out. I use the florists buckets with holes drilled in. Some are standing on soil in the greenhouse, the ones in the polytunnel are each standing in a tray, so if water does run through, then they can suck it back up. I don't leave them standing in water permanently. That said, I have a couple of blossom end rots on one plant. I think some varieties are more prone to it than others. All of my cherry tomatoes are fine, but a Shirley next to it has some with blossom end rot. All same size pot, all watered the same.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.