Thanks for the replies both! After posting this, I went hunting for a top box, but then saw a brand new humpback for half price so couldn't resist, knowing how versitile these are.
Thing is, I couldn't figure out how to get my seat off to fix the base on so I just carried on using my spinexx tail pack, which is fairly handy anyway as I can stick my gloves and stuff in the mini cargo net on it, unclip the straps and off I go. I do like the humpback though - hubby has one for his bike and I like the idea of having the base permanently attached with the advantages you have listed.
I'm in the process of selling this bike at the mo so I'm taking both sets of luggage with me and I'll get a topbox for the daily commute and keep the luggage for longer trips (like the NW200 in May!!)