Tomato Problem - what is it.

It sounds very much like blossom end rot.  Your desctiption is spot on.

Cheers,  Tony.

It could also be cat facing. BER is worse as it renders the affected bit inedible cat facing is just ugly. I'd have a Google of both to see what you have

Sorry what does cat facing mean ?   Mrs B

Thanks for your replies.  Never too old to learn.   Definitely the blossom end rot.   Thanks again,  Mrs B

Carrying on on another problem with tomatoes.  Why are some varieties of tomato still remaining green round the calix (or stem end) and into the fruit? 

Cheers,  Tony.

Could be tomato greenback?
https://www.rhs.org.uk/problems/tomatoes-fruit-ripening-problems

If you read further down that link, you will see reference to greenback, which is tomatoes stubbornly staying green around the shoulders when the rest of the tomato has ripened.

It may just be late ripening, as you say, but your original query suggested that it was more extreme than that