Thanks Noggin, good idea about using a 'normal' rucksack, I will go to millets I think and have a browse.
My original changing bag is trashed too and although it was good, it magnet-closed so you could grab things quickly and easily - very handy in first few months but useless now as T will be in it grabbing everything out too. I definitely need zips. At the moment I have a £7 bag from tesco which does the job well, but I fancy something with 3 separate pockets - one for baby, one for T, one for me I think 