Ages ago I posted asking about supportive pushchairs for DS. Glittery suggested an Ottobock Kidsert.
I didn't buy it straight away as it didn't look snuggly enough for DS. However, I bought one last week as I was sick of not being able to go on long walks with DS and it's brilliant. (I bought a different pram for going round the shops, but it's a beast to steer and can no way go on walks in our village).
The insert it isn't very snuggly, so I put a thin lambskin over the top and so now it's comfy and supportive.
It isn't an understatement to say it's made a huge impact on my life. I can now take DS on long walks and not worry when he goes all floppy. It's so supportive that DS can sit up and actually see the world, rather than being slumped in a little ball in the corner of the pram.
So thanks Glittery- I'm so glad you sugested it to me 
To anyone who has a baby with low trunkal tone I'd say the Ottobock Kidsert is well worth investigating.