Hi - I use a Recaro Young Sport for my lad - he is almost 2 now and fits in it very well. The headrest is adjustable for height and as he/she grows, padding/bolsters gradually gets removed making the seat 'larger'.
It finishes up as booster in the end.
http://www.recaro-seats.co.uk/child-...port.php#image
It isn't ISOFIX though, but is quite light so easy to take in and out if required. It secures in VERY well with the seatbelt using a clamp type fixing to hold the belt tight. Had a few of these 'type' of seats over the years and this by far excels them all.