When playing the PC, what does the Sonos controller say it is receiving from the TV? My guess is it’s showing it is receiving a stereo signal, which would be why you’re not getting anything from the surrounds.

Furthermore, I suspect that the PC isn’t actually sending a 5.1 signal to the TV either, but that’s just based on my previous experience in getting a 5.1 signal from a PC. There’s a plethora of various settings, starting with the app, and then the audio driver, and sometimes the audio card itself. While it’s been a while since I messed with it, at the time, it was challenging, and certainly the PC did not respect the CEC commands being sent from the TV set. 
 

There was a thread a few weeks ago where someone had their computer connected to their Beam. Although the computer was supposedly putting out a 5.1 test signal, but nothing was playing to the surrounds, when he contacted the computer manufacturer they admitted 5.1 was not supported by his system and he needed a different audio card. 
 

If your TV has any HD channels, try tuning to one of them. BUT make sure it’s a 5.1 DD program they’re showing - many go out in DD 2.0. Once one is playing, you can check what your Beam is receiving in the app. 

Thanks for the advice, the computer part is fine - I'm a computer engineer and as stated it works fine with my av amp (similarly connected to the arc hdmi slot, rather than looping through).

Airgetlam you mention viewing the audio being received but can't see this in the app (android) and I've been over all the options?

In System, About My System, look under the room for the Beam, there is an ‘audio in’ field.

If PC is outputting 5.1, which 5.1?  DD5.1? DTS5.1? LPCM5.1?  The Beam can only play DD5.1.  Else it might end up as stereo, or PCM2.0.  Or silence.  My best guess is that your PC is sending LPCM and the Beam is saying ‘sorry, best I can do with that is PCM2.0’.  Only a guess though.

System-About my system shows a snapshot of what your system is receiving at that moment.

What form of 5.1 is the computer sending to the TV? Dolby Digital? Dolby Digital+? DTS? LPCM?

And I thought I'd looked in that panel, totally missed it.

It thinks it's PCM 2.0 so looks like it doesn't like the DTS5.1 output from the pc (to test I reconnected the amp and definitely getting surround out of that).

So now to see if I can force the pc in to just Dolby digital 5.1 (which it says it supports) and see if I can get this working. What a faff, for the price of the beam one would expect a wider codec support, I guess I should read the fine print in future.