Thanks for the input everyone. I went for the new Enduristan Sandstorm 2 Enduro.
Rock solid on the bike and quick and easy on and off
Waterproof out of the box
Hold its shape when empty and easily expandable
Full size map pocket
Plus I emailed Enduristan about a couple of female buckles, with which I can add a couple of small extension straps that will have it functioning as a rucksack too.......and they're going to send me them FOC. Brilliant service!
I have had my eye of the Kriega gear too - thanks for that - and they just brought out a fantastic soft luggage and rotopax system, the Overlander 60. Nice......