Hi again Louise
It's a bit hard for me to give recommendations as each "case" is rather "individual" and depends on your car, i.e. the shape of the seats and so forth, and it also depends on your budget, your needs, what's easy (or easy enough) for you to install, which one suits your childs age, weight, height etc. There is a whole lot of text on this page
http://www.childrestraintsafety.com/car-seat-reviews.html that I suggest that you read through (skip the reviews though, as they're not all up yet, but the info on what you need to consider at the top is still good).
There's quite a few convertible boosters out there. I could go on naming them all day. There isn't any "one" seat I could recommend because as above, it has to meet all those requirements (good fit in the car, suits your child etc etc) because there isn't any point me "recommending" a car seat if it can't be installed properly in your car!
Some things you might want to consider however - how much does your child weigh, how tall are they, do you want a tethered convertible booster? If so, do you have approved anchor bolts in your car. Or, if you don't want tethered convertible boosters are your seat belts long enough for some un-tethered ones that require a decent amount of seatbelt length? Also, one of the biggest deciding factors does tend to be the budget, and fair enough too!
Knowing that information might help me to "prune" the list down so to speak..
