Hi everyone, thank you so much for your help and support, it's been great!
As an update, shortly after someone bought that Montana Sapphire off WildFish, I happened to make contact with Alistair Kelsey, who I also found as recommended on these forums. He is not too far from where I am in Sydney, so it's all very easy and I don't have to worry about import duties, shipping is really cheap etc. He found a oval Montana Sapphire for me that's very similar to the WildFish one, but a more of a colder, steel blue rather than light blue, a little bit smaller but over 1 ct and heated. He's going to set it in his "Embrace" setting in platinum.
@Barrett@Anne111 yes, I also (slowly) came to the conclusion that if I wasn't prepared to pay a heck of lot more than my stated budget range for a stone, I wasn't going to get something that was top colour / rarity etc etc. I'm quite happy with what I've settled on and it's been a fun journey!