Some more great options, cheers everyone. One thing that would be good is being able to use it on either bike, so that's another factor.
JB, I'm not particularly bothered about the BMW stuff, it was just my initial thought. I saw Steve's soft panniers at the Cotswold bash, and they looked good, but I just want something cheap and simple that I can just bung a few clothes in for a long weekend away. The longest that I'll be away for on the bike this year will be five or six nights, so a soft bag on the rear seat for my clothes, along with the locking topbox for anything valuable should be the perfect setup.