Ebac do a range of dehumidifier, I have a stone building north of you and keep my tools covered when not in use. Humidity is running at 75% plus at the moment.
We are in a similar position stone building (1800) cement rendered started hacking off and found damp behind it. So all the rendering will have to come off at some point. Just removing the lower section at the moment then using lime nhl 5 as we live close to the sea.
I am just about to do the pointing on our Croft plus three out buildings. Will just in a corner out off view and hopefully will get better and the job goes on. Also have internal fireplace to do. Found a very informative video on YouTube.
I just like them to be delivered undamaged, cement mixer turned up damaged (Amazon) and so did my Triton planer thicknesser (Ffx) still waiting for ffx to contact me from Friday lunchtime.