I have had a long conversation with tech support at PayPal in the US (not easy during a cyclone).
They have confirmed that my account is complete and fully up to date. They have confirmed that my browser is the correct version and is up to date.
They have tested my bank account with a test transaction.
I have tested the Paypal process with a large Australian vendor (1) as guest, so logon required at payment point and (2) as registered user with no Paypal logon required at payment.
I have tested the Paypal process with several international vendors (but I already knew that was OK).
Paypal advises that if you get your system administrator to contact them then they will take you through the process to correctly configure your system.
I appreciate that you do not want the orders that I was trying to place, but I do not understand how you can be quite so sanguine about all the others that you never knew about because they gave up and moved on to a vendor that had a correctly configured payment system.