@rinq thanks for the suggestion! Trouble is our sofa is a lot bigger than the one they already have in there which means it would pop out longer than the wall by quite a bit (so when you walk in from the hall, right to the left you will have a big chunk of sofa coming out). Then also the window is so big that the back wall is tiny so that's why the current owner has a tiny TV stand and even so it still covers the window a bit... Never thought it would be so difficult to arrange it when we fell in love with this house...!

I agree you don’t have to be symmetrical. I did a sitting room for a client recently and had to work around an off centre window and a panel heater. By the time the room was painted and furnished the asymmetrical design wasn’t obvious. What I would say about your drawing tho is that the panel round the mirror is very tight. That maybe dictated by the width of the tv on the right but if you had room to widen it, it would look better. Are you set on having the tv in the wall? Or could it be on a floorstanding unit/table/other (something sympathetic to the style of the room)?

Another thought would be to balance it visually by adding a false display of wood stacked up in a recess. A mantel shelf could then span the entire width providing a visual divide. The upper portion could take an art piece or mirror across the section with out looking unbalanced.

I too would remove the mantel if possible and either do without or add a new one, symmetrical with the surround. I would put in a plasterboard wall on the left, from floor to the level of the top of the chimney opening (black area). Install shelves above in wood or paint in a deep colour - the horizontals will optically widen the area and give a more balanced appearance. When the skirting board is reinstated it will look altogether more pleasing.