Waiting for an update after the board change.Well, the service engineer came out and I demonstrated it doing it on live TV and YouTube (Fully Charged Show ). He took one look and said "It's the main board. Seen it loads of times. Knew what it was as soon as you said the sound and video are out."
So they will replace the main board sometime over the next week or two. Optimistic? No. But happy for them to give it a try seeing as he clearly knew it was a known problem. I shall keep you posted...
After a lot of attempts I found an acceptable solution. By using arc digital output set to auto, truemotion set 0/0 an lipsync in service menu -3, it is almost spot on for all apps except youtube, it is even better than internal speakers. another alternative is auto, real cinema on truemotion off lipsync 0. Result is very similar.
Only problem is that selecting auto instead of passthrough makes sound quality worse and dts audio is limited to 2 channels.