The BBC will be broadcasting a five and a half hour live climbing marathon later without knowing whether the two climbers will be able to complete the overhanging cliff challenge on Harris in the Outer Hebrides. Top climbers Dave Macleod and Tim Emmett have been practising and checking out routes on Sron Ulladale for weeks. Now the 55-strong production team is also on site and the BBC plans to broadcast the climb attempt live from 1:35 today (Sat 28 August) on BBC2 Scotland and from 5pm on the BBC HD channel as well as being streamed live on the internet.…