I wish I'd never asked this question now. You all seem to think I'm such a bad mother . I felt quite upset about it all last night, to be honest.
I don't know why I feel I have to justify myself, but here goes.
My HV weighed him a few weeks ago and said that, being over 9kg then, he should go into a bigger car seat. MIL, also a HV, siad the same. When I took him in his existing rear-facing seat to look for a forward-facing one the people advising on fitting said he was too big to be in it and it was dangerous.
The car seat says it should be used up to 9kg or 9 months, whichever is sooner.
Most importantly, he doesn't fit in the thing. His head comes up over the top, the straps are too tight and dig into his shoulders (that was 2 weeks ago, I doubt they even do up now), and he was screaming with discomfort. Once I finally got him into the thing, he tries to sit upright and his weight and upper body strength are such that the whole thing tilts forward so that instead of sitting at a roughly 45 degree angle to the car seat he's almost upright. Probably haven't described it very well at all.
We held out as long as possible, but I can assure you that there was no way he could be transported safely in the rear one.
I'm new to this mothering lark, and I'm learning as I go along (and have much to learn), and I'm probably very over sensitive (put that down to sleep deprivation and PMT), but I was shocked that so many people wanted to tell me why I was doing everything wrong. It took me years of ttc to get ds, and he is very precious, so I'm really not going to take chances with his safety.