I.D. required Shrimp/scud

In my mates garden he has a large plastic sheet 30' x 20' on an area that was a veggie garden. Since my mate has been in the house this has remained undisturbed. On checking there is a large population of freshwater shrimp or sand-hopper type of creature. This is a population that has been living and breeding for a least 7-9 years so self sustaining. I was wondering what they might be. My first thought was freshwater shrimp as there is a stream about 200-300 yards away, but they look and behave like the sand-hoppers you see under seaweed on the beach, but i was not sure these could survive outside the marine environment.
I will attempt to collect some and get pic's ( however these shrimps all look a bit alike to me).
We are located in the south west of the UK, as mentioned the garden is 200-300 yards from a stream and about 5-6 miles from the nearest beach. Any info would be much appreciated.
TIA

Peter

Hello
They might be lawn shrimp. Google it and take a look.
I also found this Strange - Mini-infestation - Wild About Britain

Thanks for the info. I sure they are the introduced species your link highlighted, my next move is to see if I can culture them.
All the best

Peter