Crack repair

[footprints]

Hi all, I have a crack in my bathroom ceiling, I have filled and painted a number of times, but guess what it comes back!
This morning I decided to bite the bullet and cut out a couple of inches wide back to the plasterboard and insert skim before repairing.
Having started I can see someone has tried that before you can see the scrim in the photos.
No access at all to the back of the sloping ceiling.
No sign of popped plasterboard nails or flex in the area.
House built 1935 ish.
Any suggestions please or do I bodge again!

 

[footprints]

Quite disappointed no one suggested polystyrene tiles, surely the ideal solution!
For anyone in the same boat I decided to stitch screw it on alternate sides, an impact driver beds them well into the existing plaster and flakes a few loose bits off too.
Going to try some Ronseal "super" flexible filler, fingers crossed.