Originally Posted by
ringingup
If I try online, it allows me to change the inbound for a fee of £60 + £70 (for one passenger).
Then I'm afraid I don't know what sort of fare you have or what its conditions are for the inbound half. All the current R class fares that I can see, both LON-VCE-LON (which would be used as a half-round trip fare to construct the fare for a single ticket) and VCE-LON, say "CHANGES NOT PERMITTED".