GM,
You will also find that the Romer Prince, as sold by Mercedes and up until recently on the Peugot web site will also fit in the rear. This can be used with lap belts! You should be able to get one for about £80.
However, this seat only goes up to 18Kg, so you daughter will only be in it for a year or so, depending on size. My son has just turned 3, and is now borderline for this seat.
Above 18Kg, you can use one of the standard booster seats that should take them until they can use the seat belts on there own, can't put my finger on the name at the moment, but this WILL need 3 point belts. I think there is one that Halfords sell that will fit the back of an older 911.
When I say booster seat, I mean the kind that has a seat, and full back, with guides for the shoulder belt, rather than just a booster.
Of course if you want to splash out on a Porsche Prince, I believe the back comes off to leave a simple booster seat that can be used with a lap belt. I have also seen a Romer Prince booster, but I think this was designed for sitting on transmission tunnels.
At then end of the day though, with older children, a lap belt isn't good enough.