We have the Hipod, which goes to 26kg. I have a heavy boy and it's great for him (DS2). It also anchors in and you use your normal sash belt (the one in your car already). Like Feeb I have heard only good things about the Maxi Rider, I think on Choice that and the Hipod get rated the best?
We got it from Target during their sales.
I am pretty sure the new laws have variances in them that allow for Height and Weight.