A Carradice type saddlebag and a Bagman rack would be an excellent solution to your problem. I'd also use a small handlebar bag to carry your valuables that you would take with you when you left your bike. I'll post my set up again.....I'm setup for long distance touring as I have tent, sleeping bag etc, but you can adapt the general idea for really light touring too.
How have you got that sleeping bag/tent wedged in there between the carradice and the seat post? I've also got a carradice (Nelson LF) with Bagman QR support and am trying to figure out ways to carry a tent/sleeping bag along with considerable other gear (have not got a rear rack).