Just an update on this story.
I ended up importing a used booster from Europe. $100 for the part,$ $150 in postage so not too bad.
The workshop fitted it, with much cursing… , and all is good. But they said it was an old part and no guarantee how long it would last.
They gave me the faulty one with the suggestion to see if I could find someone to replace the membrane, their usual guy wouldn't touch it.
Now that the pressure is off as I am back on the road, looking for suggestions where I could have the old one fixed up?
Not is a hurry and happy to ship it around Australia if needed.