on a commercial both would be liable, thats why both get fined same amount fron H & S, to stop companies picking cheapest price and not using companies that dont take H & S seriously.
As for a domestic customer, I, and i dont say this very often are not 100% sure, but the home owner would not be liable imo, and remember the law states that is ladders are used need to be secured and afull written risk assesment done. Dont think many domestic home owners woild know how to do this, or for that matter half the window cleaners either.
idealrob