I should know the answer to this as I have 3 children....

[KTeePee]

but I can't remember when you are supposed to move them on from "proper" car seats (i.e. with a harness) to the next stage

Have always had a "trigger" before when a younger sibling needed the seat and the older one got moved up!

Ds2 is 3.5 and tall for his age. He has been asking for a booster seat (strapped in with seat belt) and I have just noticed that his head is well above the top of the seat he is in now. But if he is safe to stay in his current seat I would leave him a bit longer (he can't open the straps whereas he could undo a seatbelt)

Ds got the next stage seat when he was about 3.5. I think the car seat you have tends to do them up until max age of 4, depending on height and comfort.

As longs as the straps still fit him comfortably and come in above his shoulders iykwim, then he is still ok in his current seat

have a look here. For what its worth mine went into a booster seat with a back on it at this age, and all of them were fine. The youngest only once tried to undo the seat belt, and got such a telling off he wouldn't dare do it again. He also has his brother and sister keeping an eye on him in the back only too eager to grass him up anyway

when my ds first sat in his high back booster seat, he tried undid the belt a couple of times but was well warned that the police would come and take mummy away if he did it again
it worked

My booster seat says from 15kg, but it's old - I think they recommend not going into boosters until they are over 18kg now. DD went into a high back booster earlier this year, when she was 3 and a few months, but she's very tall and hit the weight target too.

His Stage 1 seat should last up to 18kgs or if his eyeline goes above the back , a highback booster can be used from 15kg and a booster cushion is not recommended before 22kg.

Better go and weigh him! Thanks all.

Think I will go for a high back one when i do move him on - any recommendations?

this is the one we've got. Both my dd and ds have this one and have got on fine with them. dd is now 7 and still sits in it, so it lasts well too. Hope that helps a bit.