DIY Doctor UK DIY Forums

I've just bought a 1930's bungalow. Upon steaming off the wall paper from a front facing bedroom, it seems that a lot of the plaster has come off too. There are lots of hairline cracks all over the walls. Ive proceeded to knock off more plaster with the intention of re-skimming. However, under some of the plaster is a layer of a light grey thick layer which looks like mortar?. In some places this mortar is crumbling away. Out of curiosity, I drilled a hole through the wall to see what layers were unearthed. Behind the light grey cement part, there appears to be breeze blocks.

I've attached some pictures to help identify the wall layers

Question: How many layers should we remove before attempting to add plasterboard/skim?

Thanks