In reply to DaveMinsh:
> From the guide book it appears there is just about option to abseil from the Great Ledge using my 50m half ropes. Is that correct - would I need to move towards the right hand end of the ledge?
Yes there is a pair of good abseil bolts in the centre of the back wall of the Great Ledge, from which you can ab to the ground with double 50m ropes. You need to walk right a little from the top of pitch 2 of Left Hand Route to just past the large sitting boulder.
With all these abseils (the long established ones and the new ones Rick has described), you need double ropes and you do need to be careful not to get the knots jammed. You also need to take care not to throw ropes or knock debris onto climbers below!
Incidentally, at the shared belay at the top that Rick has described, there is a second sling (without ring) to tie into while rigging the abseil.