That link is geo-restricted, but I think this is perhaps the same clip on YouTube:
Also - those guys are insane. The "safety harness" was a wooden plank chair with some ropes dangling you over the abyss.
If you think that looks dicey, search for the video of when another Blue Peter presenter,John Noakes, cleaned Nelsons Column in Trafalgar Square. Note the overhanging wooden ladder and the entire safety equipment was a pair of flared jeans and platform boots!
He also climbed the mast of HMS Ganges which is possibly even more heart stopping.