As mentioned, I have got to the bottom of this without Onkyo's help. I replaced the offending component, which is a 5.5V 0.1F backup capacitor. This is mounted on the front PCB, just below the right hand side of the display. It is a right pain to get to. Here briefly is what to do:
1: Unplug (really important!), remove top cover.
2: Undo three screws underneath front panel edge, pull off volume and tone knobs, and gently pull off metal front panel from the bottom, which is held with sticky tape inside.
3: Undo five screws from plastic front panel and free it. You will now have limited access to the front panel PCB and audio processor PCB.
4: Cut a few black cable ties as required to gain better access to front PCBs, and unplug ribbon cable from right hand side. It may be helpful to undo the nut and so release the volume control encoder.
5: Still struggling with poor access, release the standoffs from the audio processor PCB and unplug its ribbon cable. You have to remove the audio processor PCB in order to get to the screws under it. You may want to remove the PCB for holding the mains switch.
6: Now with some kind of access to the whole of the front panel PCB, undo 10,000 screws which secure it to the black plastic front panel.
7: Now with limited access to the front side of the front panel PCB, locate the capacitor, towards the right hand side of the display, just beneath it. Note the polarity; the negative terminal goes to the left/top pcb land, positive to the bottom/right. Desolder and replace this component. HINT: Make life easier for next time and mount the new part on the REAR of the PCB, so now negative will be down. Care with polarity!
8: Reassemble carefully. Make sure that the volume control encoder metal clasps are tight as they come apart easily. Fit new cable ties in original locations (important to prevent hum pickup).
The parts you will require then are about 6 nylon cable ties, and a 0.1F 5.5V capacitor. I had to make do with a 0.047F part because it was all I had to hand and I didn't want the machine in bits while I ordered one. Many of these components today are radial PCB mounting, which are not an ideal fit but could be used at a push. If you can get a vertically mounted one, that would be better. The nearest I can readily find is from RS Components item 377-350 (Panasonic EECS0HD104V), and the leads on this could doubtless be re-jigged to fit.