Update time:
The tech has just been and had a knowing look on his face when I described the issue. They have taken the TV away to test in their workshop. Depending on what they find they will either replace the mainboard and try again with my Sonos or if still not working then the TV is likely to be replaced, but once again will believe that when it/if happens!!
I supplied them with the transcripts of my discussions with Sonos and what Sonos had found in the diag report I sent. They also didn't look shocked when I said I have tried the Sonos on another C7 with the same results.
But OMG I'm now using an old 32inch Panasonic in the LGs place. It looks so small....


