I noticed that the slack in the head of the SB-910 was causing the head to drop a bit when the Better-Beamer was attached. This threw the beam below the field of view of the lens.
Not willing to open the flashgun to see if there is a way to reduce the slack, I opted for a much simpler solution.
Two layers of wood cut off a tongue depressor (approx 3mm total) taped to the body of the flash provided a shim to stop the head dropping.
The beam is now centred in the field of view.