ok, asked for you on another forum which has some experts in the field amongst its members....
Looks as if it could be a Steatoda species, Family Theridiidae. As for which of the several UK resident species you have, the pale abdominal colouration is actually somewhat confusing, because Steatoda species usually have glossy mahogany-brown or black abdominal colour. It could be that the colour has lightened because this is a gravid (pregnant) female, in which case, the most likely possibility is Steatoda bipunctata, colloquially referred to as either the Rabbit Hutch Spider (courtesy of their frequent appearance in such constructions) or the Coffee Bean Spider (courtesy of the fact that the abdomen in many specimens resembles a coffee bean in appearance).
If it's a small spider, bipunctata is your most likely candidate out of the Steatoda species, because both the other two commonly found members of the Genus, Steatoda nobilis and Steatoda grossa, are usually considerably larger thanbipunctata, though still not especially large. If you're familiar with Amaurobius spiders that frequent window sills, then fully grown adults of bipunctata are usually about half the size of an Amaurobius, whilst nobilis attains about 80% the size, andgrossa possibly attaining 110% of the size, though it''s rare to find one that large. You're looking at about a 3cm leg span forgrossa, the largest species, and no more than 1.5 cm for bipunctata.