On Saturday I cleared a bit of duckweed and stuff out of my pond. On Sunday I moved the pile to the compost heap and found a little, empty shell inside it.
It was a hinged one, very pale/almost white and shaped rather like this: http://www.seashells.org/seashells/imperialvenus.htm
It was about 1cm wide.
What might it have been?
I can't find pictures of pond shellfish anywhere on the net.
My garden is a long way from the sea!