Thanks to Jantango for the reference to the article about the shortage of bandoneóns: it is here.
I said that the Arnold family 'continued limited production' after fleeing to the West: in fact they only tuned and sold instruments, so there were no new instruments after 1950. The full article from Todotango is here.
The film, El último bandoneón, is on DVD but not in a PAL version, and so not available in the UK. It is available through the US Amazon.com and could be shipped to the UK, but it's not that cheap. If anyone sees it and recommends it, I'd be glad to hear. If there's good footage of the older generation of dancers, I'd be interested.