Thanks to you both for that advice, the picture speaks a thousand words, Dave and you have confirmed my suspicion that the step has to be removed to adjust the “back-stop”.
My comment on the load on the rig was because I intend to use a muscle box (=magic box) fixed alongside the centre board, for the genny halyard so the pull on the sheave plug will be kinda 45 degrees upwards and I thought it would lever out the plug, but of course the pin in the mast step will counter this, so that should be ok.
Thanks