PVA is the latex in acrylic latex paint.
Poly(vinyl acetate), or PVA for short, is one of those low-profile behind-the-scenes polymers. I mention PVA glue as asking the 12 year old manager at B+Q for a polymer may throw him somewhat.
But if you're happy paying for expensive products, go...
I had similar and the solution was simple and cheap. Buy some 'good' PVA glue. Mix about 2 tablespoons to a pint of water and paint the whole wall (after the plaster is dry). Let it dry over night and you will not have any issues with plaster sucking out the moisture, cracking etc.
This also...