@ZIGGY - based on an article from the express and echo dated jan 14th 2009, at the time, there were 14 black swans.
I believe the numbers were even greater in years gone by, but due to dog attacks, pollution etc their numbers have fluctuated. The good news is some that are bred here are sent else where. This is due to their territorial nature and as such the Brook can only accomodate a certain number of breeding pairs safely.
I've only lived here 14 years but I do remember counting 18 black swans once, probably just after they'd successfully bred.
As OLD FART says they are sometimes sent elsewhere when there are too many of them. Don't know whether money changes hands for that. I think it's also possible that we take in swans from elsewhere (maybe Slimbridge ?) when our numbers get a bit a low, or to add genetic variety.
I've tried several times to find out more but our council's website is utterly unforthcoming about the swans, strange when you think how much public interest there is in them.