Solid wall insulation

[billybuntus]

But what if the fundamental design of the building is flawed?

I concede some properties may have design flaws built-in but I think your assertion that an attempt to cover the flaw with a waterproof insulated layer will solve the problem takes a lot of believing.
Questions come to mind such as where is the damp in the wall coming from and where is it going to go eventually.
If you are proved wrong and water does get in, which I think it will, the "improvement" exacerbates the situation considerably.
Meanwhile, even if you are able to construct a completely waterproof layer that lasts 50 years, where is the water in the wall going to go in the interim?

But if the solution lasts 50 years its a definite improvement on the current situation?

The damp in the wall is coming from the fact that I have a single skin extension built on top of a rubble filled stoned wall. It was always going to be a recipe for disaster. Add to that a 4 inch step forcing water into the render covered stone and it can't get much worse.

The water that is currently in the wall will eventually balance out. Its not the perfect solution but I don't actually think there is one other than starting again or just living with it.