Thanks for all the tips HotI - the mothers choice one at Target I think was the Nobel - people seem to think can get 3 in a Yaris - and they did look pretty narrow prob 45cm I reckon. As does the Maxi Cosi Goliath like you say.

Once a childs shoulders meet the height marker does that mean they should definitely go to the Booster mode - I put DS in DD's seat tonight and he was maybe half an inch off the height marker - but in the just boosters are the height markers at a different height? It seems a bit silly to buy a convertible booster for DS if he would have to go into booster mode very soon.