Why some trades use tanking slurry before plastering?

You are spot on my friend, damp proofing nowadays is more to do with covering up and sealing in the problem than trying to solve it. By the sound of this, it's not a rising damp problem but a penetrating damp or a condensation problem. Without being able to see the problem it is impossible to be 100% sure what is causing your issues. The type of building you have is a solid twin wall construction, built using lime mortar, it would have had lime plaster on the inside, finished with a lime wash. This would have allowed the building to breathe, most older buildings need original materials in order to work properly. The problem is that most tradesmen do not understand how lime mortar, plaster, render, works let alone how to apply it. The type of materials required to correct your issues are still available although a lot more expensive, but without actually finding the source of the water ingress it will be impossible to correct. Good luck Alex