Ryedale in Scotland!
Nay lad. tis Yorkshire like.
OK, as a quick explaination, the difference between modern buildings and old buildings in terms of damp is that modern buildings are in effect a waterproof shell. No moisture gets in. The materials are mainly waterproof. In an old building by contrast the materials are breathable. Water gets in but also gets out again. So no DPC, no concrete, etc. This does mean that you may get a little damp in an old property, but combining an old breathing building with modern waterproof materials can cause serious problems because it stops the building breathing.
By putting a waterproof concrete render on the outside of the wall you stop moisture evapourating from the wall, so you might get noticable damp on the inside because its the only way for it to go.
One thing you need to be VERY concerned about is Cob. DO NOT put a dpc or waterproof materials over cob. Cob has to have a certain level of moisture in it to retain its structure. It is after all only soil. Very dry soil is a powder. that isnt very strong. Cob is fine as a building material, but dont let builders who dont understand it anywhere near.
working with lime is very easy (easier than gypsum plaster) and you get (imo) a nicer finish for an old house.
I would strongly recommend ppuk
http://www.periodproperty.co.uk/discussion_forum.htmThe lime plaster on the inside will have been done with a wooden float and perhaps finished with a thick limewash.