Blossom End Rot (BER)

I've only seen it on pot-grown beefsteaks and plums, probably because of erratic watering or nutrient imbalances, and I've not had any problems with BER since I started growing in the ground.

I doubt it's a lack of calcium in the soil, unless you frequently see signs of calcium deficiency in other types of plants.

Here's some good information from a university website (https://hort.extension.wisc.edu/articles...m-end-rot/):

"Blossom end rot is caused by a lack of calcium in the fruit.  This lack of calcium may be due to low calcium levels in the soil.  More often however, there is plenty of calcium in the soil, but its availability for uptake and transport to fruits is impaired.  Drought stress, alternating soil moisture extremes, and damage to a plant’s roots all can inhibit calcium uptake, as can waterlogged or cold soils, and high concentrations of ammonium (NH4+), potassium (K+), and magnesium (Mg++) in soil.  Movement of calcium within plants depends on active transpiration (i.e., loss of water through above-ground plant parts).  Because leaves transpire more than fruits, calcium moves more easily into leaves where it remains.  Calcium is not later redistributed from leaves to fruits.  This preferential distribution of calcium to leaves can be made worse by over-fertilizing with nitrogen which promotes excessive production of leaves.  In addition, high relative humidity OR low relative humidity in combination with hot, windy weather can limit transpiration, thus preventing calcium from reaching fruits."

"Avoid conditions where there is too much or too little water.  Water evenly and mulch the soil to retain moisture during dry periods.  Avoid practices that would damage roots (e.g., cultivating too near plants thereby cutting roots).  Use nitrate (NO3–) rather than ammonium (NH4+) forms of nitrogen fertilizer.  DO NOT over-fertilize. "

Informative, I'll just see that watering is consistent and hope for the best next year.  I fed seedlings early on with miracle grow with a bit of epsom salts and after fruit set with bought tomato fertiliser. It will be interesting to see whether any further fruits suffer from it. Watch this space.