In reply to Tony the Blade:
I've used the very back wall of Horseshoe a few times. The best line, I think, is just to the left of Excavator - it's almost always empty, comparable pretty solid (though helmets still extremely advisable), and has trivial walking access as you can just go round the side above the Main Wall and Upper Tier to get there.
You will ideally need two static ropes, though - there are two good trees to rig from but they're a bloody long way back so you'll use one 50m rope just for the anchor, and as it's a long abseil they'll be far happier going down on a second static than a dynamic.
Oh, and be prepared for the system to shift a considerable way towards the edge when they weight it - I once set this up for some children with my personal tie-in near the edge, let a (rather heavy) dad have a go at the end, and nearly got pulled over the edge myself!
Jamie