OK, here's where I'm up to at the moment...
...all ready to kick over!
The problem is it doesn't want to fire up. Not even a hint of catching.
Which leads me to suspect the Spark or Fuel issues.
There is a spark (though I'm not convinced it's a particularly healthy, fat one so it could be failing under compression). It's got a new ignition coil but it is the original generator in there, and I've read the recent posts on the main XL600 thread regarding the expected output from the ignition side of the generator. I haven't checked that yet, but I will do so when I have some time.
And, for the amount of kicking over on full choke I gave it, I'd have expected the motor to be flooded and the plug to be soaking wet with fuel - which it wasn't. I've cleaned and inspected the carbs, paying special attention to the pilot/idle circuit on the primary carb. All the jets a clean and nothing seems to be blocked.
There seems to be good compression, and though I don't have the tools to hand to measure it, I've no reason to suspect problems in that area.
Any ideas folks? Is there something vital I've missed?
On the plus side, I am developing some excellent muscle tone in my right leg!