Thanks Steve and you have pretty much nailed it there. You can get it for around £499 at some retailers which i think pitches it more at the Sonos Beam market than the Arc and a more attractive price point.
Now the firmware is here and i have it paired with the Sub and 2 x 150's, i am very happy with the system and it sounds fantastic.
I already had the Sub from owning the Denon S716, but if i was buying all from scratch and wasnt already invested in the Heos ecosystem then the price is very off-putting when you consider what 7.1/5.1 systems you can get, with only the compact size of the 550 being the advantage against bigger bars.
Apart from the cost, my only gripe is not being able to Airplay or play vis the app Apple Music to multiroom grouping on the Heos or paired systems, like 2 x 150's or the 550.
Maybe that's coming in a future firmware update, or possibly not. Not sure if its a technical issue or the legal agreement after the dispute with Sonos.