I ended up making anchoring points in the rear floor area to suit, and using the rear seat backrest cross member for the upper fasteners. The 3 year old has now outgrown the reat seat 12 years on!
Thanks, having read that there are mountings included I am hoping to use those and not drill anything. It will be very occasional use for my granddaughters so I don't really need to do it, but would love to take them out in the car.