ISOFIX seats are pricier than the conventional child seats, but its a price worth paying as ISOFIX seats remove a lot of the doubt about the correct fitment of these seats.
I've been using the seat belt type seats for 3-4 years, but having recently purchased this Britax one, I can see straight away the massive difference it makes (especially in a 2002 BMW 3 series). Once fitted it doesn't budge at all, and is so easy to move from car to car. The stickers on the side of the seat tell you if you've fitted it right which really helps.
Shop around on the net and you'll find this seat for around £150 (I used babys mart who are highly recommended)

I find it hard to fault the Britax Duo Plus Isofix car seat, and was shocked to see one reviewer advising not to buy it. I bought this for my son when he was ready for the next step up from his infant seat (usually at around 9 months but he was closer to 12 months to meet the weight guidelines). I wanted an isofix seat and after alot of research opted for this seat (purchased from Halfords). I agree with some of the comments that it can be initially hard to fit but I think this is more so due to the cushioning in the vehicle seats which surround the anchorage points, having not been used before they needed a bit of stretching. Over time it has become much easier to fit and my son looks very comfortable in it. The recline is good in fact I keep it permanently in the full recline position which he seems to like and his head always falls back or to the side if he falls asleep in it. All in all a great seat, in fact I am going to use it for my daughter in the coming months and move my son up into a Britax Evolva 23. Highly recommend.

DO NOT BUY A BRITAX CAR SEAT. It's a strong brand name but don't be fooled. The locking mechanism is very poorly design and you will have extreme difficulty getting this seat in and out of position with any speed at all. It's fine when it is in position but the manufacturers have clearly not considered that people do need to swap cars seats from time to time - and do it in a minute or two without the aid of a mallet. I gave this one star only because there was no option for zero stars.

A really great seat and once it's fitted you can be totally confident that your child is secure. The harness could have a bit more length to make it easier to clip in before pulling the adjuster to tighten. I wish they made them in a bigger size - My son has just about out-grown the Duofix and I'm nowhere near as confident with the Recaro Monza Seatfix that he'll be using from tomorrow!