I mentioned that someone on AliExpress was selling Jinhao X750 pens with the Majohn flex nib fitted, but while I was thinking over that Ali decided I needed to see a listing for various colours of X750 on sale. So the purple one for £3.50 went in my basket, and it arrived today. I've just pulled the flex nib from the Jinhao Dadao 9013 where it had been not very successful and fitted it to the X750. Success, of a sort. Still needs the feed to be reprimed every few lines, but it is actually possible to get words out of it.
Unfortunately I don't actually like it very much. Without pressure it's just a scratchy EF. With pressure it's a scratchy F to M that runs dry almost immediately, and I still can't get any real line variation with my writing style. It might be me, or it might be that the nib got damaged during various taking out and putting in operations, but I think at this point I'll write it off. I did buy another nib as a complete nib unit, which I will put in the parts drawer until I feel enthusiastic about dropping £20 on a Majohn C1