Late to the fray... but here in the U.S., your problem is known at Blossom End Rot... similar to EcclesCakes answer. However, don't destroy the blighted tomatoes just yet. At your local plant store, you'll find a product that is a calcium solution, usually in a spray bottle. Liberally douse the tomatoes with the calcium for a few days running... we've had good to fair luck with that treatment here in the U.S.
The problem is caused by over watering or, more likely, uneven watering... letting the plant dry out almost completely and then dousing it to near drowning. Tomatoes like the soil to be consistently damp but not standing in water.
Best of luck!