Just to answer my own question above and maybe help anyone else searching down this same road in future let me outline what worked for me.
I decided to use the Zone 2 analogue L/R phono outputs into the corresponding inputs on the (Legacy) Sonos Connect in the Living Room.
I had setup the Line-In some years ago and it still reflected the Pioneer SC-LX85 as the name of that particular line in so I renamed it.
I selected Line-In with the Sonos app for the Kitchen (Legacy) Play 5 as the input and selected the Marantz Living Room as the source.
I then used the HEOS app to select a track from Amazon HD and the app started to show the track playing. Went into the kitchen...no sound. Turned the Sonos volume to max...still no sound.
OK...back to the Marantz and figured out I need to have Zone 2 turned on! That might help.
Now I have sound in the kitchen. But...and there’s always a but...not only did I have volume control from the Sonos device & app but also in the HEOS app and the Marantz volume control from the front panel, remote control & Marantz app!
Much too much and after going into Settings/General/Zone 2 I was able to change from Variable and choose a fixed output setting (I settled on 75 to balance the Sonos volume with other sources) and figured I had it sorted.
After some more experimenting I found that as long as I left Zone 2 switched on then even with the Marantz SR8012 itself powered down in its usual standby mode I could still choose music through the HEOS app and play it through any of the 7 Sonos Zones around the house.
The only visible clue that Zone 2 was active is a small cluster of LEDS in the porthole that from a distance looked like it might spell Zone 2 but with closer inspection with a magnifying glass is really just a cluster of LEDS.
And of course I can choose any source for Zone 2 independent of the selected source playing in the Main Zone.
You know the more I dive into this Marantz the more I like it. Doesn’t sound too shabby either...
Think my next move will be to purchase a decent Bluetooth transmitter, connect that to Zone 3 and drive a pair of Bose 700 headphones.
My biggest concern is getting a decent BT transmitter. I’m looking at the Sennheiser BT-T100 currently as I don’t fancy one of the strangely-named Chinese devices but I see any mention of AAC?
Only wish Bose had equipped the 700 with aptX or aptX HD but then again I can’t have everything I suppose...
—=Glyn=-