In between helping Norman Haddow teach the students on the beginner's wall (which you can see through the opening in the photo above) I was able to come over occasionally and help out on the arch too. Kathy and Steve who run the centre have many pallets of stone and piles of interesting rocks to choose from which we found useful when it came to looking for material for the arch. It's like a grocery store for masons.
We found some big chunky voussoirs and a lovely huge stone that I shaped for the diamond shaped keystone. It took quite a bit of chiseling, (which generally I don't do a lot of when I'm building dry stone walls) to dress the stone to make it into the correct V-shape for the top of the arch.
I have a new blister on my hand now to show for it .