Be aware that unless they are approved under the new legislation it will not be legal to put children in them.
If they are approved then they will have a label on them showing the approval - off the top of my head it is something like EU 44.03, Im sure you can confirm that with a bit of digging.
I've just been through this for our Volvo with 7 seats (the rears are child seats), but in trying to find the real status I spoke to police, vosa, volvo and the dept of transport, got 'dont knows' from most - finally ended up talking to the man who was responsible for drafting the the legislation. Its a bag of worms.
eg the merc E-class has a seven seat option and the rear seats are NOT approved. So only adults can use them, like anyone would fit. unless boosters are fitted - which would make it legal, but merc say dont use boosters with the fold up rear seats.
time for a T4 or T5 ?