In reply to Fraser:
I just checked the BMC Guidebook and it suggests 20m, which sounds overly generous, but is probably about right.
It's quite deceptive actually, as the bottom/top are easy, so you cover the ground very quickly - hence lose track of how far you've actually travelled. I'd imagine it's around 8m of difficulties once you hit the groove.
All of this reminds me that I really need to go back and finish it off, it's so bloody good...but inconveniently difficult...and scary...
On second thoughts, I've remembered why I haven't been back to do it.