Problem solved. In a weird way, but it's solved so hey ho.
For some reason, the very left needle hole just wouldn't accept the overlocker needles and I expect I wasn't getting them inserted properly. The overlock needle shaft seems too wide for the hole, somehow. I kept wiggling it, and it would get totally jammed. No idea what's going on in there, at all, but it wasn't for using the proper needles (all the other needle spaces are happy as larry with serger needles, it's just the furthest left one being a pig - it's hardly used so perhaps it's rusty lol).
In desperation I changed to two brand new normal sewing machine needles, and they slid into place without any problems, worked perfectly and I now have a beautiful coverstitched hem.
There's nowt as queer as sewing machines, sometimes....