Well someone put me onto Apex car rentals and I can hire a Mistubishi 4WD Outlander SUV for $30 more - so have changed the booking, this way we dont have to worry about space and its cheaper than the other rental places. (they dont charge for car seat hire so it came out a much better price)

Tragedy averted, thanks for your input. I know Mum wouldn't mind but want her to be comfortable... (and we have to fit a pram and portacot in the boot too)