Well, Qobuz customer service have applied the code to my account, but they didn't tell me how... I'm guessing that means there isn't a way for users to do it...
I can now stream in hifi and hires. BubbleUPnP doesn't seem to want to auto advance to the next track in a playlist, so I've installed the Qobuz app on my old Android 4.4 media box. Working okay. May need to try wired ethernet, as there seen to be short disturbances with WiFi (even though the signal and data rates are good).
Library is strangely patchy, with albums missing from some artists I'm interested in, even though they're on the same label... Can't get The Unthanks' 'Magpie', for instance...
But I still cannot find any way to enter my payment details through the app... I'm sure my old details will have expired, so good luck to them when they try charging in three months time...