I have come accross the English Folk Dance and Song Society website pages. [ UK based I think] .
The official site I believe; very interesting and full with historical images, and copies of old mauscripts/tunes etc..
Very interesting with mentions of instruments [ concertinas included] and nice layout to read. Lots of images, historical manuscript images..
I am NOT an employee of them, but I am aware of the Socety historically [including Vaughan Wiliam's/ Cecil sharp connection etc]..
Have a look for it yourselves? It is a very interesting and useful resource.