So, I had a plastics appointment today and got the situation sorted out. They were hoping to put the implant straight in but because I am at the extreme small end of the market they didn't have a range of sizes available as they normally would & the smallest was too large so they put in the expander as a temporary placeholder. It seems like a custom implant will need to be made - I have another appointment in a couple of weeks when we will look at what size will be needed.
The part that had annoyed me was why didn't they just have the correct size implant ready beforehand. But the registrar explained about how they need to get the exact right size and made it seem reasonable that they have to do it in 2 operations to be able to get it right. He did acknowledge that it was a shame which was nice.
The part that confused me was the people who I asked about this kept telling me the expander was to expand the skin. The registrar clarified that in my case the expander is not being used to expand anything, it's just keeping the place ready for the permanent implant.