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It looks theridiid like but I can't personally say this is S nobilis - legs look too delicate and annulated. Can you get a picture from above, to show any markings on abdomen and cephalothorax?
I still lean towards a theridiid, though 8mm is big. I wonder about Achaearanea tepidariorum which is one of the larger members of this family. Was it found in a warm place?
It was in an insulated outbuilding. The photos of Achaearanea tepidariorum on the web (no pun intended) certainly look like it.
I don't think it likes having its photo taken.
Lat/Lng: 51.3033, -0.3452
OS grid ref: TQ154573
In an outbuilding;