Thanks.
I went to cut the grass near the pond at the weekend, and tried using an old bedsheet to cover the lawn - anchored by bricks. Whilst it worked in as far as keeping the grass clippings out, it sucked up a load of water which came with it when the sheet was removed. In future I think i will be getting a butterfly/fishing net to scoop out and clippings after moving. It does seem the easiest solution.