This might be helpful for when you're wearing that thing out-and-about Gareth...
I was in my local bike shop a few months ago. The Ryders sun-glass rep was in there stocking the rack wearing a similar bag. I look at him, smile, and say "nice man purse", he turns reaches down holds it up and says "Indiana Jones had a bag like this"...
In fact ...... Indi hadn´t.
He used British Gas Mask Bags MK VI and MK VII (depending on movie and scene, mostly VII) from WK II. They replaced it´s original strap with a leather strap, and he was ready to do some action with it.