Thnx for this. Maybe I had them too close together. I watered them evering evening - about two pints per plant. I did this because I understood if they flopped that would cause blossom end rot.
As a precaution I intend to spray the whole greenhouse with Jeyes fluid in the early spring
rocketman, if it was Blossom End Rot, it's caused by plant stress. The plant, stressed, is unable to distribute sufficient calcium via its internal mechanisms to the fruit.
Planting too close together won't trigger BER. It's more likely to create a climate for fungal problems with a lack of air circulation.
It's often said that an irregular watering pattern is the major cause of BER, the irregular pattern stressing the plant. It will certainly cause BER. As can fluctuating temperatures, excessive heat, etc. As can overwatering. I think you were overwatering. I'd follow Dovefromabove's watering pattern posted above. Less is always better with toms, both water and fertiliser. Toms perform at their best when left pretty much to their own devices.
It's hard to know what the problem was at the stem end of the fruit. Have you already disposed of the fruit? Is there a chance of a photo?