This is a mature male exampke of the Noble False Widow, Steatoda nobilis, the species principally implicated in last years media reports. That said there is no need to panic as 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 bee or 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.
Could you tell me where you have seen this by forwarding a grid reference or post code to me at email@example.com 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.