We have just put DS into a stage 2/3 seat this week and he's 4yrs 8 months! We got ours at Halfords, they have some good deals and an extra 10% off if you buy 2 on line.
I had a chat with the guy that fitted it for me and his advice was to go by weight rather than age. DS has only recently got to 15kg which is the lowest weight for the stage 2 seats, DD will probably hit 15 kg before she turns 4 so will be in one earlier than DS is. They need to be 15kg for their weight to work with the seat belt, but if they are too heavy for the car seat I was told the seat will just break apart in an accident. The seat he is just out of could have gone up to 18kg, but it sounds like he is better out of it before he hits that weight.
I had a good look around and there doesn't seem to be any stage 2 seats with harnesses in the UK, but I wouldn't use a seat without a back as they just aren't tall enough for the diagonal part of the seat belt to be in the correct place. The seats with backs have impact protection for their backs and heads and you can pull the back up higher as they grow, there is a guide for the seat belt which keeps it in the right position across their chests. If he is not at 15kg yet he is better off in his stage 1 seat with the harness until he rechers that weight limit.
Laura