Hi Neil, Hydrocotyle ranunculoides is a huge problem in the US too, many lakes have signs asking for boaters to check their boats for the plant so it doesn't get spread to other lakes
The Ash disease sounds really bad...I remember the American Elm disease (from Detroit MI, we lost so many trees in just a couple of years, streets that had a lovely canapy before hand, never looked the same afterwards!)
Take care,
Cheers
