Thanks for your speedy reply.
The pond is in hyde, cheshire. Its about 4 years old and lined with rubber. I've added nothing to the water. no heavy rain recently so not much possibility of anything leaching into the pond. Haven't tested the water for nitrates, nitrites or ammonia. Someone suggested tonight that the deaths could be due to decaying plant matter producing these chemicals and it sounds like you are suggesting the same thing.I do have a u.v.and bacterial? filter and pump which I installed last year to combat an algal bloom. Not used it this year yet
I'll test the water as soon as i can get a test kit and let you know the results.