Terrestrial amphipod similar looking to Sand Hopper.
No interactions present.
I doubt the ID could be confirmed from this photo as amphipods including Talitrus need really careful examination to be sure of species, but based on the location info in your other posting http://www.ispot.org.uk/node/293663 (in compost well away from an obvious freshwater or marine source), I think this ID is correct and a mild area of Wales is a likely location for this Aussie intruder.
Nick Upton, naturalist and photographer.
Lat/Lng: 51.6833, -4.9495
OS grid ref: SM962024
Damp garden.Shady spot 3metres from damp remains of former garden pond and in damp humus beneath 2 month old garden rubbish. 30 metres above sea level. 250 metres from tidal water.