In reply to Flinticus:
Yup, the rock arch is at the top of Dexter/Main/Right-hand Gully and the usual route is to scrabble your way up underneath it. It is in a bad way at the moment. Was up there last Saturday and there was a fat bloke jammed in there, which probably won't do it much good either.
The last few times I've been up there, I've avoided it by taking a line up the left wall of the upper part of the gully, once past the steep wall in the gully bed. A little ledge traverse then takes you across to the exit point of the arch. Much better.