That's a great document, thank you for the link Leo. Though I just find it hard to actually believe that adding 100mm external insulation to 2 ft thick stone walls is actually going to do any good on a "Type 1" house like ours...
As per @candor's point above, you need to be careful with old stone dwellings. However, if you can envelop the walls within insulation and address bridging, the walls effectively become a thermal store, slowly heating as your heating system runs and later slowly releasing heat as the house cools.