In the end we opted for the Deluxe kanga-roo from Wee-ride.
Not cheap at £100 but they re-sell well on ebay.
http://www.weerideuk.co.uk/Weeride_Deluxe_ltd.asp
Front facing center mounted, mounts onto a bar which bridges headset to seatpost.
Tried it for the first time this morning round the park and it was brilliant.
I was happier as I could see him and make sure he was secure.
The seat and harness is fantastic, and he was happy.
I still feel safer with a front mount for now.
Let's face it, if a car hit me, it's most likely to be from rear or side in my opinion. I can get my arms round him and fall with him tucked up in me.
As he gets older and stronger I'm sure we'll switch to a good rear seat.
All riding until he is in a rear seat will be off road anyway.
Green route at Llandegla - here we come!