if the flats are as cold as i remember them in scotland, get a hanten 丹前（たんぜん）or dotera どてら（褞袍）.
http://www.e-miyata.com/winter2000/hanten2000s.html
there are cheaper ones, don't worry. the local futun shop around the corner to me sell them for 1,700 JPY i think it was.