Rotting peppers

I have three sweet peppers grown from seed in my greenhouse. They are fruiting well but when they are about the size of a hen's egg, the base of the fruit turns brown and when cut open the lower part is rotten. They are watered sparingly and frequently and the greenhouse is well ventilated. Help Please!

 

Peppers like warmth, cold nights can really play havock with them. Other than that I cant think of what might be wrong.

 

I sometimes had the same problem.

I think its similar to blossom end rot in tomatoes.

The other cause could possibly be that the plant is trying to support too many fruit.
Thinning the fruits to just three to a plant might help.