If you live towards the south of the UK, most likely cause is insufficient water - the fruits will need a fair amount of water to develop properly. They must have had some because they're still on the plant, haven't shrivelled and dropped off, but if you are in the south, its probably worth making sure you give them plenty. I'm assuming they're growing in the ground though...

If they are just growing to a certain size, then turning black from the blossom end, then that means that they haven't been pollinated.