tomato problems...grrr!

Hi there,
I am currently growing a mix of tomatoes in a greenhouse. Due to the patchy summer this year they have been particularly late in fruiting and ripening. I am having a particular problem with the sungella plants in that the fruit that has come, is rotting where the stem, (pedicel), enters the fruit. It's showing itself as a developing circle of light grey colour in the fruit. There is no visible mould or anything, rather the fruit itself turns grey. This spreads and the fruit becomes soft and rotting, usually dropping from the vine. This is very frustrating as the otherwise perfect looking fruit laying on the ground is often the first sign of any trouble. Any help would be greatly appreciated!