We were in the same position not so long ago - found that pretty much all seats fitted. Did what alltorque suggests - camped out in Mothercare trying out different models and ordered it for £30 less on line!T:
Fits fine on the double and single front seat, but as said above in the middle seat its a bit hard to get second gear.
If you have rear belts worth checking as sometimes the belt isn't long enough to fit round.
Pretty sure it's Cosytot seat thats the same with different trims of for quite a few of the britax pushchairs so you might be ale to find one of those to try instead??