By the noise they were making I would say there were hundreds of these about in a small area of unmown grassland.
No interactions present.
The song (stridulation) is likened to a continuous electrical discharge. Very distinctive once heard, and now you'll probably hear them everywhere (as I did in Oxon in the mid-1990s).
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
Lat/Lng: 52.340556388889, -1.5887783333333
OS grid ref: SP281714