Filling gap at corner of two walls

[JamieB3009]

I have an area of plaster near the corner where two walls meet where a strip approx 8mm wide is loose. Ive raked it out and it appears there is scrim tape behind there, but the plasterboard looks to have been cut too short when installed (see photos). What would the best way to fill this be to prevent it cracking again?? Would injecting expanding foam into the gap to provide a support and then cover with polyfilla be best?

Thanks.

 

[Wayners]

PVA ( wood glue will do) 50/50 with water. Lash that in gap.
When dry powder fill.
Problem is the movement if any?
Yeah. Could use foam then fill.