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I still don't know if it was the right thing to do, and there is just no point in worrying about it, now it's done. I'm not even really sure what the other options would have been
We had 'damping proofing' done too a few years back. I suspect it was over expensive for what it's was. However, once the walls were tanked and what not, there wasn't anymore damp on the inside, so I think it kind of worked.
Addressing the cause of the issue would have been much cheaper; after all, it turned out to only be a missing bit of pvc pipe. However, we only managed to find it a few years later once the whole kitchen was ripped out, including the entire suspended floor and the patio outside. This obviously wouldn't have been an option if your only aim was to 'sort out the damp'. Even if you could have diagnosed it, you couldn't have fixed it without being really destructive.
Personally I suspect it's probably not the right thing to do, probably is too expensive and sold by cowboys, and I can easily imagine that you might just be storing up damage in your walls for the future; but it's not always possible to find the cause and (in our case at least) did remove the damp in the interior walls and plaster and bought time until we could address it properly.
This is no real help to you but we were in a similar position, and we got DPC done. We were a bit clueless and It was only after we had it done that I saw people online saying it was a load of rubbish peddled by crooks. The house had all of the windows painted shut, no heating apart from 2 gas fires, has no damp proof course and the joists are straight onto soil in a lot of places.
I still don't know if it was the right thing to do, and there is just no point in worrying about it, now it's done. I'm not even really sure what the other options would have been.