Ed, it's a small silted up pond, approx 2 x 3 meters.
The swan mussels (Anodonta cygnea) are found locally in slow running rivers, lakes and ponds. I believe that the European bitterling (Rhodeus sericeus) uses the mussel for part of it's life-cycle.
Over here the freshwater pearl mussel (Margaritifera margaritifera) is I think the most endangered. It is protected under Schedule 5 of the Wildlife and Countryside Act 1981, which was strengthened in March 1998 to give them full protection. It can live for up to 100 years, only thrives in clean water and requires the presence of wild salmon to reproduce.