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Here's my attempt at inserting an image of the room with crude diagram of what I'm working with. (NB the little cubby hole mapped into the diagram where the L front speaker is doesn't exist it's wall all the way to the outside wall)

Wires are in the wall and ceiling already so I'd be happy to cut open and it will probably make my life easier to fish out the wires. The rear 2 are on an external wall with insulation behind so not sure how that affects installation of in-walls, maybe I stick with on wall for the rear.