Shrimp are excellent scavengers, although I've no idea what's available in Britain for shrimp food.
I really just wanted to mention for anyone thinking of putting a freshwater crab in a planted tank, I've just recently seen a shocking! (lol) photo of the results in a once-heavily-planted Walstad, although there may very likely be kinds of crabs not so interested in expensive aquatic plant salad mix.
(It just seemed rather rude to put that in with no response, however useless, to the OP's question - sorry, joshjames)