Hairline Crack in new Ceiling Skim

[mcprinter]

I had an Upstairs Room Ceiling and Walls skimmed about 1 Week ago.
Top Windows left open at the Front and on Vent at the back. No heating.

We have been here 20 Years
Ceiling was papered. When I removed it looks like it used to be 2 Rooms.
There was a 6 inch band of Pinkish Gritty 'Plaster' between the 2 Rooms. The old dividing Wall.

The Crack looks like it is at the Joint of the Gritty Plaster to the old Ceiling.

1930s Lath and Plaster. One of the old Rooms was Over-Boarded and Skimmed before we moved in. I had to cut some out due to previous Water Damage from Chimney Breast
Not sure what the finish of the other Room was.

The Plasterers have done an excellent Job.

Not sure if I need to call them back in or it is something I should sort.
Guessing Filler would just crack again.
I have Toupret and Easifill. Also have some Paper Tape and Fiba Tape.
Fill with Caulk or something else flexible and paintable?

Thanks

 

[Wayners]

Get up in loft and look.

Maybe plasterboard in the channel where wall was removed and that's solid, but the lath plaster is flexing a bit causing crack.

You can vac. Pva. Then pour plaster mix from loft side to strengthen loft side of crack maybe. Will stop the flex.
Hard say without looking

 

[mcprinter]

Get up in loft and look.

Maybe plasterboard in the channel where wall was removed and that's solid, but the lath plaster is flexing a bit causing crack.

You can vac. Pva. Then pour plaster mix from loft side to strengthen loft side of crack maybe. Will stop the flex.
Hard say without looking

Thanks Wayners
Will have a look in the Loft Tomorrow

 

[mcprinter]

Nothing obvious crack wise in the Loft.
There is no visible difference from above where the 2 Rooms used to join.

Is it still worth doing the PVA and Plaster? I already have PVA an old Bag of Multi-finish in the Shed.
PVA neat or watered down?