Many thanks - I called and surprisingly the availability was good.
Booked HKG-NRT-HKG in October 2019.
IB website showed zero availability on CX
IB call centre showed good availability and I was able to complete the booking over the phone
Their computer systems are slow though ... took a long time to display the flights and to calculate the Avios needed
But I was able to get the award issued, so I'm happy!
(the call took 30 minutes...)