Many thanks for the replies and suggestions
I think it looks like Blossom End Rot - looked up some images on Google when Fleecewife suggested it as a possible cause and it looks spot on.
The internet blurb says it can be caused by calcium deficiency - the leaves essentially taking the calcium preferentially from the fruits so we are going to remove some of the leaves from the plants to see if that makes a difference.
This is the first year we are using an automatic weepy pipe watering system - 2 lots of 10mins every 24 hours so the watering is consistent and certainly everything seems to be thriving