I spotted this quite large freshwater mussel in the shallow waters at the edge of a lake in Norfolk. Beneath the gunk it looks like it's probably quite a rich mixture of yellow/green/brown and I suspect it's a member of the Unionidae family as they look very similar. I don't see them very often, though apparently they're very common, especially in canals, though I remember finding a very large shell (perhaps 15cm) at the edge of a lake as a boy.
Update: thanks to Matt/Matthew who comments that it could be a Painter's Mussel (Unio pictorum) and I think he's absolutely right. Apparently the Painter's Mussel is so called because it made a perfect receptable for artists' paint!