Hi John and welcome, I look forward to seeing photos of your car.
As a suggestion, Halfords offer a trial fitting at most of their shops, as do Mothercare. Both have a good selection of seats and should be able to try a few to ensure the correct fit.
As the 4-seater rear seats sit higher than the front row, making them prone to more wind buffeting, I would avoid an Isofix seat that requires a separate base unit (the seat clicks onto the Isofix base, rather than having the unit integrated) and try one that is 'all-in-one'.
Another options is BMW/Mini/Porsche all sell the same seat and while it was pricey, I found it to be an excellent seat for my daughter - easy to install, easy to put the child into and excellent safety rating.
I've posted a photo so you can see what I'm talking about: -
They are available direct from any BMW, Mini or Porsche dealer and only the colour options vary from brand to brand (the seat is the same).
You can often get them on eBay, new, for reduced prices - most dealers don't stock them thus if someone orders one in and doesn't pick it up, they often end up at cost on eBay via the dealerships 'shop'.
