I exchanged a few e-mails back and forth with Qatar, but ultimately wasn't in a position to be entitled to claim compensation:
We have further checked on your feedback below and found that there was indeed a schedule change that occurred on your itinerary from Doha to Singapore.
Our records show that QR942/29 June 2022 had been canceled since 01 June 2022 and the alternative flight on QR946/30 June 2022 was automatically added to your booking. Further to our records, the cancellation was done on your itinerary on 08 June 2022 and was automatically notified to your respective booking office/agency.
In view of your claim under EC261 regulation, please allow us to inform that as the schedule change occurred more than 14 days before departure, which was notified to your booking agency (Qantas), we regret to advise that we are unable to offer compensation on this occasion.
And so, the fault as so many things are these days lies with the Red Roo who never contacted me to inform me of my change in itinerary (the flight was a classic rewards booking through them). Had I known in sufficient time I could have got a PCR test and been able to avail of a hotel in Doha at Qatar's expense. I was QF'd, and not for the first time this year. They currently owe me $400 in additional charges for a cancelled rewards flight and have adopted a policy of ignoring all my communiques in the hope I'll just give up. Absolutely awful company if things go wrong.