Got a FL Vrs Octy at the moment and things are getting a bit tight in the back for the kids.

Our clan consists of an 11 year old not in child seat, 4 year old in isofix booster with back and baby who soon due to go into second size seat.

Is the yeti any wider across the rear? And if so by how much?

less than 1.25m across the seAT PADS  -  I have a class m r/c yacht in a trnasport box the above length, can't shut the doors when its laid across the seats

Don't forget, it's not just the width but how close the Isofix brackets are to each other. E.G. in a 3 series BMW they seem closer together than in Yeti. I can just fit between 2 large child seats in my Yeti. Suggest you find a friend with a Yeti and try your seat/booster cushion/11 year old combination in the back. Alternatively, you could try it at a dealer but my daughter tried this recently and the VW salesman was understandably a bit impatient.

Love my Yeti but you may find it less 'luggage friendly' than Octy.

With a growing family if you are worried about space you might want to go and check out the cargo area of a Yeti - it's not spacious and if you have a spare wheel (which you might want to do with a young family rather than potentially be stuck by the roadside for an hour or more if you get a puncture) you lose some of the height benefit. For me its not a problem and I was happy to trade space for some of the Yeti's other advantages (such as just being great fun to drive) but it certainly has the smallest cargo area of any car I've had since the Ford Anglia 105E, noticeably smaller than Peugeots 405 and 406 and a Volvo V50 and I believe the Octavia estates have more space in the back than these. 

Have you seen the space in a superb estate?