someone wrote:

................................. but you may be able to get some vouchers
which you can put towards some quality paint with a lot of pigment in it.

S'far as I can tell, it's not the depth of pigment that's an issue here,
but adhesion. The PVA's had the predictable effect of sealing the
surface of the fresh plaster, making (as he reports) the subsequent
paint lie loosely on the so-sealed surface, and bubble/peel off when
sneezed at (or as local evaporation of residual water in the fresh
plaster pushes the PVA-and-paint or paint-where-PVA-didn't-cover off the
wall) - a problem which is likely to be made worse, not better, by
either putting on yet more coats or covering as B&Q suggested with an
oil-based paint, thereby making it effectively impossible for any water
to diffuse to the surface.

Hence the dismal pragmatic solution of lining paper. The OP's enthusiasm
for scraping off the PVA-n-paint layers is understandably
infinitessimal, as would be their chance of doing so without putting
gouges into the luvverly smooth new plaster finish :-(