55h 29m 45s!
If I had bothered to set up render layers I think it could have been in half the time or less.
Ah well, live and learn as they say. The somewhat extreme time is down to the fact that all cloud layers are set to receive shadows and there is a light source inside the ghost cloud.
The only part I'm not happy about is the lower half of the reflection, still very noisy.
Enjoy and have a great and spooky Halloween or Samhain or whatever you celebrate
Richard