Your room is not ideal. The Arc relies on a flat ceiling to reflect audio from the two upward firing speakers. And the long distance between the Arc and the left wall will severely effect the Arc’s left surround speaker.

Thanks. Would the Trueplay app mean it could compensate for the sloping ceilings? As to the left side, if I added sonus ones to the left and right of the sofa at the back would that solve it? 

Hi @snapperjonno 

Your room while not the recommended dimensions should yield some of the benefits a Arc surround system (Arc, Sub and surrounds) can offer.  It appears the initial distance between your Arc and ceiling is within spec. In fact the gradual incline of the ceiling at the point of contact while not flat doesn’t appear to be too drastic as the sound would be directed down to the listener just before the ceiling apex.  

Adding surrounds would compensate for the increased distance between the side firing speakers of the Arc left and right. The sub would complete the package.  Depending upon budget you can add surrounds and sub later. I might add the surrounds first...JMO.

Yes, TruePlay is always recommended.

Take a read at the information provided in the link.

Arc Placement | Sonos Community

Thank you for all the info, that's very helpful and much appreciated . 

If I were to build a cabinet with sides that enclosed the Arc on the left and right effectively creating 'walls' would that work for the left and right channels? If so, how far forward would the sides need to come from the front of the Arc? 

If this doesn't work, 'm getting the feeling that for the money I should perhaps try and find a better solution for this room, which is a pity as I love the simplicity of the Arc and its Sonus integration, and the fact I can add to it, and because its wireless... 

Any pointers for alternatives?!?

PS I seem to have somehow set tho s post as an answer to my question by mistake, can I reverse this?