Thanks for the replies, all good info, just a further update/query.
I have attached a picture of the wall in question which may help. I think the plasterer was saying that the polyfill was required to paint the wall where the lining paper was removed as the existing wall is either a concrete...
I have just finished getting my plasterer to infill a doorway into my living room with blocks and then had it rendered. Previously I had studwork and boards in the infill but there was some cracking so decided to infill with blocks and replaster.
However the living room has lining paper...