Does anyone know why my Aubergines are rotting on the stem before they mature?

I haven't grown aubergines before but I suspect you are over watering?

I have an aubergine and it is currently 50cm tall , with many leaves and a few small fruits , which are yet to grow . Mine is positioned againt a wall so its gets the sun for about half of the day , and we give ours a fair amout of water ...

These plants need a lot of sun, and love the heat. This is the second year I have been growing Eggplant(as we call them here in USA) and they really don't get growing well until the daytime temps are consistantly above 80 degrees F. Mine are getting 6 hours of sun and I have now harvested 3 fruits.
I would be concerned that yours are not getting pollinated inside the greenhouse. Hope this helps.