Courgettes, or Zucchinis,as they are known here in the U.S. are susceptible to blossom end rot, just as are tomatoes. This is caused by a lack of calcium. Even if you are not overwatering the soil can be calcium depleted. Add some dolomite (common lime)to the soil or the remaining fruits can be sprayed with a liquid calcium mixture available at most garden stores. Additionally, since you are growing these in a greenhouse, you may have a problem with the plants not being pollinated. Zucchini must be pollinated... as you probably know, the plant produces both male and female flowers... if you don't have pollinating insects, usually bees, they will produce fruit with the condition you describe. Hand pollinating is a good answer to the problem. Simply pull the flower petals off the male flower exposing the pollen... rub this on the interior of the female flowers, et voila... baby zucchinis... Best of luck!