A moisture meter is a handy thing to have for the workshop, but they can be a bit misleading in houses. For a start, this is a long-term thing, and spot checks, unless you are keeping some sort of graph updated, can cause unnecessary alarm. BTW, wallpaper may not be helping your problem. Ideally you would have a stone wall plastered with lime, and then limewashed, allowing any moisture in the wall to evaporate slowly over time. Is this damp patch behind furniture? If so, relocate your furniture because airflow is ultimately what will sort this problem out.
If you resolve the ground level issues, and if you fix any leaks etc, then you won't actually have dry walls until after the coming summer. So long as you are aware of the sort of timescales we're talking about, then by all means stick a moisture meter on it.
I'm a bit puzzled by the airbricks. Could you post a photo or two? Stone buildings universally have solid ground floors in my experience, so an airbrick is pretty pointless unless they are servicing a fire or a boiler (or maybe a larder).
You sound as though you have paving hard up against the house. When you come to assess your floor level/ ground level relationship, have a look at that paving and see if it could be moved away from the walls. If you can get it a foot away and can fill the gap with shingle/ gravel then that will help a lot. The further the better.
As you're walking around the outside, constantly ask yourself "if I tipped up a bucket of water here, where would it go". You do get the odd drop of rain in Cumbria (I don't know if you'd noticed
), and it really musn't ever be allowed to pool against the house.