if the customer has not asked for them, then he is ultimately responsible. if he has asked for them, then you as main contractor are responsible. however should an accident occur, the pooh will hit the fan.
the person who employed the injured would be asked for risk assessments & method statements (even if he is self employed), if he doesn't have them, then you would be asked for them, then the customer would be asked. if there are none, then you are all liable under due diligence. your insurance would not pay out and the injured party would be able to sue you all personally.
the actual h&s safety tree starts at hsaw 1974 all the way down to acops, everybody at work is encompassed. if your employing a company or person to chuck em selves off the side of a building on 2 ropes, then you need to make sure they are irata or equivalent, i think there are actual laws covering rope work and there should be minimum of 2 people on the job, a level 1 supervising a minimum level 2. google irata and they have website with all the necessary info.