Well I finally got around to making the set up I need.
I used a different pack in the end that was a bit less fancy looking but had a better capacity.
15000mAh at 5V. A darn sight better than the internal 940 mAh battery that's for sure.
So far it is working well and the capacity of the pack far out weighs the additional bulk in your pocket.
I cut the end off an old USB lead and added the correct round plug to go into the Fuji adapter.
The lead is robust and long enough to be threaded through my outer clothing to a warm pocket within.
I also have a smaller solar rechargable pack which seems to work too that might be useful in a last pinch.