Hi
This issue has been solved, by a Mark a chap who has been working in the electrical retail side for years.
My sound bar is a Panasonic SC-HTB690.
I was told, eventually after a few attempts to wire it as follows.
From the Virgin box connect a good HTMI cable to the HTMI In socket of the Soundbar. ( my case this reads AV IN, BD/DVD.)
From the television, Connect a good HTMI cable into the HTMI ARC socket and then put the plug on the other end into the HTMI OUT (In my case this reads AV TV ( ARC )
Other equipment can go into any other available HTMI slot. As you can see, In my case, I've connected my DVD player to the televisions (DV1) HTMI socket and my Amazon fire stick HTMI 4)
The result of this is that I no longer have to manually change the source to get the device I want, as they now connect automatically and the Lip Synchronisation, works perfectly every time.
What you may find, as I've found with mine, is that to watch recordings the soundbar has to be set to HTMI ( or in my case BC/DVD) Whilst it's connecting, you may find that the TV keeps trying to connect to the Bar, So press source again. This time it will stay on HTMI or in my case BD/DVD and after about twenty seconds. You should get sound through the soundbar. At least, I hope you will, it did in my case. You will also find that the Television screen goes dark for a few seconds whilst the source connects.
You will also find that when you want to retune to your Virgin box, after watching another device, you will have to do that manually as for some reason, the Virgin box, won't do this automatically.
Don't shoot me if it doesn't work. It worked in my case.
Hope this and the diagram help.
Best Regards
Peter