OK, so some progress...
I can get audio out of my amp using both sets of the following settings (please see below), one with 'Bitstreaming On' and one with 'Bitstreaming Off'. (I've no idea what is technically being output with 'Bitstreaming On' but I am getting audio out).
However... for both sets of settings there is a problem with the surround settings as I seem to be getting the same channels across all speakers, as opposed to the speaker specific output for centre channel, or rears etc etc. (Previously this worked properly using the bitstream method - I've no idea what has now changed). My amp says it's receiving a multichannel 7.1 input and I am testing using Multichannel SACD tracks.
1/
Bitstreaming/Yes/HDMI
Audio Device/HDMI [Core Audio]
Settings/DSP & Audio Format/None
2/
Bitstreaming/None
Audio Device/HDMI [Core Audio]
Settings/DSP & Audio Format:
Output Format: Enabled and Processing 96khz 64bit 8ch
Output Encoding: None
Channels: 7.1
Sample Rate - All are set to output 96khz
Note: Using this option, my Mac Mini's 'Audio Device' settings has reverted to the Built-in output, the HDMI output can no longer be selected.
P.s. Many thanks for the heads-up on setting output to 96khz Brian. I'd tried it before on the 768khz setting and it didn't work. But when I applied it to 'All Sample Rates' it worked