Managed to get the C14 shock fitted today, will post some photos later.
The top mounting was a bit of a problem. The metal sleeve through the C14 bushing is 2 mm longer than that on the C10, so that needed filing down. As mentioned above, the same sleeve should ideally be drilled out to a 12 mm diameter. But I don't have a 12 mm drill bit, so instead I modified a couple of washers with collars which I had lying about to allow the 10 mm bolt of the C14 to fit snugly in the 12 mm hole through the C10 frame.
I fitted the shock with the bike on its centre stand. Rather amusingly, I had to let most of the air out of the rear tyre to get the dogbones lined up with the holes on the linkage!
With the shock fitted, it's very very easy to move the bike onto its centre stand. In fact both wheels stay on the ground, The side stand is way too short now though, I'm currently resting it on a 2" thick piece of wood.
One question : if I compress the shock spring, does anyone know if the lower shock fork can be turned round 180 degrees relative to the top? The adjusting hose comes out on the left-hand side of the bike and it might be better if it was on the right.