So, I've over 400,000 Avios, and I wanted to book a business return for two to Sydney, Australia, some time in July or August.

This looks impossible. Indeed, there are no business seats available this year, except January.

So: do additional seats become available? Or am I basically stuffed, and I should simply pay the money (which I'll be doing on Star Alliance instead, since it's considerably cheaper).

And does Avios + cash make any difference to the availability?