I'd just waft a filling knife, float, or whatever over, with some
ordinary filler, over plaster - however, the OP said "Hairline cracks
have developed on the ceilings, where the plasterboard panels meet.".
The benefit of squirting in PVA is that it sticks the lot together,
not just the edges of the PB - it glues PB to joist, too, which
re-inforces the joint considerably. I've used it doing odd jobs in
lets, and on P&L ceilings - it really is much better than just
filling
the cracks, which otherwise just open up again in big oblong patterns
in no time.

A problem with flexible filler is that it can "shine through" a
couple
of coats of emulsion. You can't really sand the stuff to feather it,
either. However, I use quite a bit of it - the Ceramide stuff is
good.

How about if I used some artex patch filler?

http://www.artex-rawlplug.co.uk/artex-360

It would match the existing ceiling artex OK. But would the crack just
open up again in time?

Bruce