hi shelagh,
ive got the mothercare catalogue which makes it easier to understand. it says rear facing car seat up to 13kg, then theres the forward facing car seats for those over 9kg. it suggests that the baby reaches the recommended weight and that they can sit upright.
there is a combination car which is from birth to 18kg, it sits rearward until over 13kg then can be positioned to forward facing once baby is over 9kgs up until 4 years so this may be another option for you. gosh its so much to take in. hope this helps a bit.
alix