Thanks Kiwi Flyer

If I understand correctly then I have two options;

Option 1 - I buy a 5 continent RTW ticket from London and don't take the flight home at the end. This would mean buying an extra return ticket from SFO - BKK (because I need to visit Bangkok towards the beginning and end of my trip)

OR

Option 2 - I buy a Circle Pacific ticket from Thailand, and then buy a seperate one-way ticket to get there.

After having another look at the plans we don't need all of the flights listed, and finishing in Sydney would be sufficient. We can then arrange our own way back to Bangkok from Darwin through Indonesia and Malaysia. This would also save on backtracking from Singapore or Bangkok. I'm not sure if that would have a massive effect on the mileage used (going straight from SYD to BKK and not taking the flight?), or maybe open up some other options?

In terms of booking a RTW or Circle Pacific ticket from Thailand / Vietnam, is it simply a case of going on to google and choosing a travel agent over there that offers these fares, or is there another way of doing it?

I've been looking into this for a while now, trying to find a relatively inexpensive way around it. I've tried calling and emailing agencies and airlines, but I never seem to get an advisor who knows enough to help me properly. I didn't realise that going to S America would cause such problems.

Thanks again for your help