I'm not sure that the spider and the dark grey blob are associated.
No interactions present.
It has a pale stripe down the centre of the sternum (plate below body between legs). That narrows it down to 2 species, incl. common T.extensa.
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
...associated with the spider? There were two of them in the same area.
Lat/Lng: 51.7987, -0.0053
OS grid ref: TL376129
On a reed in an area of marsh.