In reply to TeeBee:
This is one of the finest bits of kit ever made by anyone:
http://www.rockrun.com/rock-run-hipsac-classic/
I've had mine for nearly 20 years, during which time I've used it for almost all my running and for keeping stuff in on every foreign trip I've been on. It shows very little sign of wear and is very comfortable and adjustable. Takes spare top, windtop, hat, gloves, headtorch, phone comfortably and, carefully packed out of sight behind you, allows you to get up to 5kg extra weight onto any flight. It is truly awesome.