jan olieslagers schrieb:

-) where can I find a complete and authorative lsit of the Q-codes to be
used in aviation R/T?

I don't think the official documents are on the internet, but there are
a couple of private sites with lists. A German one:
http://www.htc.ch/de/der_Q_code.htm

-) who is the owner/maintainer of this list?

The ITU (surprize!), and for the aviation related codes the ICAO (again,
surprize!).

I think there is also a range of codes which is deliberately not defined
and free for national use, but I'm not sure on tis.