You got an absolute bargain at A$148 - worth every penny... ok cents or whatever it is.
I am not surprised you got a different answer from the UK office. I find that each time I call - even the same office - you often get a different answer to the same question. The trick is to keep calling until you get an answer you like !!

You should be able to check online to see if your seat is confirmed in PE and select your seat. However, when the lady told you it is $148 for you she may have omited to also say 'plus $148 for the other passenger' but I find airline staff seldom join the dots together.

Best PE seats are 18 - loads of leg room but usually only available at check-in.
I prefer to choose a middle isle seat as if I go to sleep the middle passenger can get out the other side without disturbing me. All the PE seats are nice but avoid the back row as they are next to the toilets (the PE cabin on A340 from SYD has private toilets). The window seats have limited leg room under the seat in front as there are 'equipment boxes' whatever that is supposed to mean.

I do think you are a bit mean not upgrading your fellow passenger though!!