Sick Goldfish - help please

Hi, I'm a long term lurker but now need a bit of help.

I have a small pond in the UK.
The small goldfish have been quite happy until about 2 months ago when all
bar one began to die.
All displayed what looked like white 'dust' on their scales and their fins
began to rot away.

I bought about 5 more a couple of weeks ago and now they're beginning to
show the same symptoms. We lost one very small fish today.
The one that was left over from last time again seems unaffected.

There are some pictures here http://urlsnip.com/123647

If someone could help diagnose and suggest a treatment I'd be grateful.

Thanks

Sam