Looking at post #22, it's strange that the worst damp is on the inner leaf, and not directly on the outer wall. How much lower is your inner level, compared to the outside steps. French drains are good if the inner and outer levels are the same, but they don't always work very well as they can get silted up over time, and sometimes, a drain channel works better. The other issue with a french drain, is you've got to be able to get down to earth level (ideally with no clay) so that the water can drain into the ground; if you can't do that, then you need a pipe with holes bedded in gravel to take the water out to a drain somewhere. Have a look at the
paving expert website for ideas on french drain types.
As you're lower on the inside in some places, you'd need to go down quite a way to make sure you get past the inner ground level, and then throw either bitumen or synthepruf the outer wall, and then concrete with a waterproofer added to get you up to the drain channel height. You may also need to hack off the plaster internally and inject a silicon cream dpc, and then tank as well, but it's odd that the ooutside levels weren't considered when the extension was done, but it's possible that the paving was done afterwards, and that bridged the dpc.
Moisture meters don't always tell the true story, but they do let you see the highest and lowest wet points, and that helps determine the worst areas, and even a wood meter will help you do this. And in addition to dealing with the outside levels and drains, you may end up tanking the walls as the only way of dealing with this, and putting down a butumen type dpc, and then adding self leveling compound on top of it.