In reply to Cheese Monkey:
That sounds difficult.
Poss use slings to escape the system/belay.
In the process tie-off your partner so that he goes no further.
Use the same with prusiks to traverse along the route,making sure of the protection as you go and if unsure back it up with new pieces.Get to place where the last functional protection is.
Make a new belay somewhere here by placing more gear to make the anchors as solid as possible.
Tie off your partner again from the new belay.
Traverse to the old belay again.
Deconstruct that belay.
Carefully move back across to the new belay,removing gear en route and depending on circumstances self belaying.
Double check or even treble check that you are well anchored at the new belay.
Put your partner back on belay.
Carefully lower him to the ground or a safe place.
Depending on circumstances swap rope anchorage for sling anchorage whilst re-arranging rope for abseil.
Either place a large knot on the end of the rope that cannot go through the belay plate, or preferably leave yourself on the end of the rope.
Abseil down yourself by still being able to use both parts of the rope.
Care with counterbalance.
It does seem rather a long process,but so far I have not figured another more succinct approach.
It all depends on lengths of lowering/abseil and the type of ground too.
Anyone else?
Rob