It's caused by the plant being too dry at the roots and thus giving it the inability to take up calcium.

Increase your watering would be my suggestion.

And add a little Calcium to the water. I bought a large packet some time ago for treating Bitter pit in Apples.

it's not a question of adding more calcium, OB, it's down to the fact that if the plant is dry, it can't take calcium in, no matter how much is in the soil

We have had this even when the plant was well watered. This was because the pH level was wrong. We cured the rot for that season by spraying the plant with Calcium carbonate and limed the soil for the next season.

Where would I buy the calcium please