Problem with 6 1/2 month old 'slipping' in seat unit

[GingerDoodle]

Hi all

I have Mutsy 4Rider. I loved it as a pram (carry cot nice and high up, brilliant suspension and great to use); since DD was 3 1/2 months we have mostly used the car seat out and about as we moved her out of the carry cot into her cot.

I was really excited to move her into the seat unit at 6 months as per the instructions; it lies back almost flat anyway and has a huge hood, sits well at tables etc and DD can now sit up pretty much unaided. BUT she just looks too small for it; the shoulder straps are a little too far apart and when sat up the actual clasp between her legs is a good 4 inches from her front and she ends up slipping down if that makes sense.

It also feels like she is quite far away / low down (but that might just be me).

For comparison I sat her in a mothercare orb which she seemed ot 'sit' in far better.

I don't really want to have to get a new buggy, as i haven't found any with a chassis / wheels as good as the Mutsy's but I am worried about her back being unsupported.

Any advice to solve the problem or a buggy recommendation would be gratefully received!