Thanks all.
Did some playing around on the website, and figured out the cause of the flight fare difference. When getting the flights changed, I'd asked to extend the trip from 9 days to 12 days, and that changed the rules.
Seems there's a switch on the rules >10 days, so that has solved the fare difference problem.
Spoke to a really helpful lady, who is going to investigate with regard to the change fee. She asked me to email in the screenshot from MMB showing it as $275 per person, which I've done, and will call me back once it is resolved.
Fingers crossed it will all work out
