I took Dave's advice and popped into a Qantas Travel Centre here in Sydney. Well, they were fantastic: told them of the changes I wanted to make, they also noticed the fare calculation omitted details and said they would investigate it overnight. I went back today, they had indeed done so, and were able to reissue the ticket with my required changes. Could not have been more helpful.

The only issue was my embarrassment at having forgotten about the two stopovers rule in the continent of origin. Having stopped in London after CAI-LHR, I was planning to fly back to London at the end of the trip, stop, and then take another weekend away in Europe later in the year. Clearly, my Europe segments are now being done as a mileage run!

Anyway, thanks for the advice - and I am very impressed with how QF handled it.