Hi Jeremy,
My local shop stopped stocking Rab recently so I took the opportunity to try on the Photon Belay when I was in Edinburgh at the weekend.
The jacket fitted very well and as I was buying a new mid layer I was able to match up the two for size.
On getting home I weighed the jacket (stated at 710g in medium on Rab site) and it turned out to be 880g in a Large.
On the end sleeves of both arms there is no insulation in back of the wrist area, this does look like a design feature.
If I had realised these two things in the shop I problay would not have purchased the jacket, as I purchased it heavily discounted and posting it back to Scotland would be expensive I am going to hld on to it an try it out.
Cheers,
Stephen