This is an example of the Noble False Widow, Steatoda nobilis, the species principally implicated in last years highly exaggerated media reports. This is not a 'deadly' spider by any stretch of the imagination.
They are known to bite and generally this is no worse than a wasp sting; however, although regularly reported the likelihood is still very low. You are still far more likely to be stung by a wasp.
My rule of thumb is that I am happy with these living in my garden or on the outside if the house, any found setting up home indoors, or wandering males, are tipped outside.
For hopefully reassuring information on False Window spiders please see the following link:
Could you tell me where you have seen this by forwarding a grid reference or post code to me at firstname.lastname@example.org as it is valuable to know where these are being found so we can get a better picture of the current UK distribution of this species.