Thanks to Michael for reminding me to actually go & look at a suitably modified rear sight. I'd forgotten, & he is quite correct; the underneath face of the bottom horizontal 'bar' of the sight leaf is indeed relieved as shown in the attached photo, although on mine the cursor has also required a little tickle as well, so my offer of a spare cursor to GeeRam still holds, if he feels he'd like one.

Sorry it's not a great photo but it was taken in haste way after my bedtime.