Ceiling crack repair help

[ceilinghelp]

Please could anyone advise me on how to repair this ceiling crack myself. The plaster has swelled in a couple of places. we are hoping to sell the property in the next few weeks and I would at least like to do a repair and paint for the process. it is a 1940's house, plater and lathe

 

[Wayners]

If it was a bedroom you can prop up with wood and boards so ceiling don't fall.
Vacuum dust from loft side.
PVA
Pour plaster and spread.
When dry remove props.
Ceiling becomes solid so you can make good and paint.

If that's a downstairs ceiling all you can do is put lining paper over, but that ceiling don't look going and probably best not go disturbing it