What we used in the days before we used wfp was a bit of 4 by 2 which was about (I think) 8ft long which was straped across the top of the ladder with 2 proper ratchet ties.
It made the ladder a bit arkward to put up but not impossible and the wood went across the whole window resting against the bricks either side of and half way up the window.
I don't know if that's acceptable in today's health and safety conscious way of doing things but that's how we done it for some jobs with the same problem.
Maybe I shouldn't have posted this.