My needle holder has arrived safe and well, so fingers crossed it helps me. Thank you all!
I genuinely am puzzled by the "not fitting in the hole" problem - the needles go in all the other four slots, but not the far left one. To be fair, it's the least used position so perhaps it NEVER was the right side (ie a manufacturing problem) but I've never noticed. I may indulge in replacing the needle bar at some point. I've just replaced the presser foot lever as the plastic had gone brittle and snapped, so the next thing will probably be the needle bar.
Overlock needles don't go in, but plain old Schmetz sewing machine needles do. Go figure. I'm just going to use these, as my coverstitch function is happy to work perfectly with size 90 universal needles but not the proper ones!!