Thanks to all for your help.

Unfortunately I'm still losing fish, another one died tonight.

I was particularly concerned about the theory that it could be dropsy. from
what I read this is incurable. Would it then affect all the fish in the
pond? Is there anything I can do?

I did add three new fish about a month ago, but that was after a two week
quarantine period and all three seemed OK, although two have now died.

Thanks again

Any further help would be appreciated.