I finally bought a nappy bag!
I ended up getting the Storksak Claire in black. It's a shoulder bag which can easily convert to a backpack. It looks stylish, is light weight, has plenty of room without being too big and a separate compartment for me leaving the main comaprtment for Tarko's stuff.
It's a little different to what I was originally looking at but it's just so practical that I couldn't go past it. A shoulder bag and backpack in one - that will be very handy I'm sure.
Thanks for all your advice girls. It really helped with the final decision!