Comment on Damp Proofing a Basement by Adding a Damp Proof Membrane (DPM) by Carey Keates.

I think I have just read a horror story – and you sound so pleased with the “result”!
Firstly, you think you’ve saved money by not using a specialist. Maybe you have, if you don’t count the hours spent on it by your son. But have you made it damp-proof? I doubt it.
Then there’s the below-ground room. Yes, you are quite right, if the walls are below ground, they are subject to water pressure. Painting some stuff on the inside may work for a while, but water pressure will push it off in time, probably when it will be least convenient. (Such coatings are designed to be used when water is on the inside, forcing the coating onto the wall).
My advice: swallow your pride, put your hand in your pocket, and employ a reputable firm of damp-proofers (check that they are members of the BWPDA). They will not only do the job properly, but should also give you a guarantee. Then enjoy the (dry ) house, and relax.