Our son has only ever been in an 'S'. We use the Mamas and Papas "Aria" pushchair and the BMW rear facing car seat in the back. With the pushchair in the boot, we can still fit in his bag (full of nappies, clothes, bottles and toys). It takes a bit of getting used to moving the seat in and out of the back, but its easy once you've got the knack. It is also a good work out for your tummy (Great when trying to get back into shape!).
I would suggest that you go to your dealer and take a look at their car seat, and see if it will work for you.
One bonus is when you go to the shops and park in the parent and child spaces you know that no one is going to hit your doors.
Abi
