Mrs Fox, we had a similar issue as you, in that DD needed to move up a car seat, forcing DS up. I have the Britax kidfix, DS is very slight for his age, was also about 15 kg when he moved into it at 3.5 and was very comfortable in it.
Whilst all the advice given above is helpful, cost is always a factor.
I recently had to buy a new car seat after DH was involved in an accident and didn't want to buy a class 1 seat for DD2 (nearly 3) who is also small and slight. I bought a mamas and papas seat that is a booster and also has a 5 point harness., but to be honest, it isn't isofix and the back falls off the base when you move it.
This is a similar car seat to the one above, with the pad. It costs £35 more than the britax at kiddicare, but I don't know how it performs.
www.kiddicare.com/webapp/wcs/stores/servlet/productdisplayA_822_10751_-1_14574_205984_10001_14626
Which best buys include cybex Juno fix (about £185) and the kiddicare guardian pro (£190).
I paid £180 for my kidfix, but I know the prices have one down now they have some competition, the normal one is now from £100, whilst the side impact one is £122. (According to Which)
If I had the money, I would be tempted to go for one with the extra piece, to improve safety while DC I small. These weren't available when I made my choice.
I'm keeping DD2 in her class 1 as long as possible, DD1 (now 6) wants out of her kidfix, but she's staying in it until she is bigger.