Well I had a look at the pack in Mountain Warehouse, it seemed rather bad really, so I decided that if I want a new pack I should just wait and get a proper one, like the OMM that Peter mentioned.
When I got back I spent ages fiddling with my old Karrimor and I've been able to improve the fit considerably. It has an internal frame so the hip belt on it is definitely useful. In fact, I've decided to try and modify this old bag to make it a bit more modern. I can add, for example, some handy hip belt pockets: http://www.backpackinglight.co.uk/product291.asp?PageID=98 and possibly put a buckle on the ends of the straps to make a chest strap. Does anyone know if it's possible to buy detachable padding for webbing hip belts? If I could add that I think I could really give this bag an old lease on life.
So my lesson from this is that getting the fit right by fiddling is vital and never discount some kit just because it's old!
Thanks for the help, everyone!