Luckily I don't have parrots feather in my pond, but I have a constant battle with azolla.
There's quite a good article in Gardening Which at http://www.which.net/gardeningwhich/campaigns/pondlife/killers.html

Here is a link that might be useful: article on invasive pond plants

have a look at Myriophyllum brasiliensis, its a mini parrots feather.

The ones on that Which list that always baffle me are the Water lettuce and Water Hyacinth as being potential problems, I've never managed to overwinter them, even indoors, so how do they survive in the wild?

I took a trip down the Nile (as one does) and there was a real problem with water hyacinths there as they multiply rapidly and essentially clog up the system. They even have special boats to pick up the water hyacinths in bulk. But that is a different climate where they can grow all year and are kept warm--not the problem here! They were beautiful in bloom though, and the smell was lovely too. (By the way, has anyone ever managed to get one to bloom?)

Maybe in the deep south ie Cornwall there is a chance they will survive over winter, or in sheltered ponds. They DO multiply very quickly in the summer, but I have never had one overwinter, or flower. I don't bother buying them now- I prefer my native frogbit which reappears each year, and I wait in hope for it's brief flower.

Interesting point re water hyancinth - how does it overwnter here. My parots feather had no problems though - it survived thick ice for several days two years ago
AnnieO

Thanks for the information on native plants, as that is really the way to go. Each year I have put in the water hyacinths and the salvinia--both don't bloom but get bigger in interesting ways, and they don't overwinter. But if they did bloom and overwinter they would be real problems. I have been quite fond of them, but I am re-thinking about next year as really, their proliferation in this country shouldn't be encouraged.

Sadly, the yellow globules ate the frogbit and water soldiers from last year, and this year the water starwort fell prey to it. So I will do my homework on native species and build up the numbers.