I just looked through a couple of books I have on plant diseases that focus on the UK and nothing specific for Eucalyptus. I wonder if it is a generalist problem, or if the tree is declining and a fungus taking advantage. Does a qualified tree surgeon there have to just be skilled in taking them down/pruning, or do they have to have some knowledge of pests and diseases? If the latter, could you call and ask what specific disease he/she thinks it is?
Another option would be if you are an RHS member, because they have a diagnostic and advisory service which, as far as I recall from when I was a member, is free to members. If not a member, might be worth joining just for that to get a prognosis. You can email them pictures of problems as well as send samples if they need them:
https://www.rhs.org.uk/members...
