You're right that it's caused by calcium deficiency but as the RHS says:
"It is very rare for soils, growing bags or potting media to actually lack calcium. There is almost always plenty in the soil and indeed within the plant. However for calcium to reach the parts of the plant that are furthest from the roots there needs to be a good flow of water through the plant. The fruits are distant from the roots and do not compete as well as the leaves for calcium, leading to a local lack of calcium at the ends of the fruit."
The solution is to water regularly, and not let the soil get dry. I find that the first fruit get it - because the watering has been irregular in the holiday periods - but the rest of the crop are fine.
I hope it got better for you too!