Shiny black spider no bigger than .5" in body, has brown markings on top of abdomen.
No interactions present.
Rather vague row of white spots in the midline of the upper abdomen, suggest this species. 12mm would make it huge, more like 7mm would be a large example.
Lat/Lng: 51.3671, -0.1052
OS grid ref: TQ319648
Tenth floor flat, kitchen ceiling.