The Mentz/Tempest Select guide says that double ropes are required and shows in the topo that this is a 40m rap. This is incorrect - it is a 30m rap (as stated by the NBF in their "replaced anchors" list). My 60m had ~2m spare on each strand.
Even slightly "short" 60m ropes may suffice if you swing to higher ground up the gully and/or land on the initial easy flake and solo down it (although beware that the roof leaves you hanging a bit out from the wall). YMMV but with a normal 60m you don't need to lug a 2nd rope up there.