You can put the TV over the fireplace, not the best situation but the way the room lays out it is the only logical place to put it. If you could you should add one additional layer on your mantel that could extend it out another 4" to deflect the radiant coming from the fireplace. I would also mount it on a slimline mounting bracket and have the bottom of the TV no further than 4" above the mantle. Ideally you want the TV to be in as straight a line as possible with your eyes. You don't want to look up too high at the TV, it's like being at a sports bar. Also if you can snake up the power and the HDMI from the cable there is a box they make that has both in one so it makes for a very clean installation.

Oh, okay, may need to clarify some more. If you get a new 2 seater in place of the 2 white chairs, fine, but no blue chair at all next to the 'new' 2 seater. Won't be right with a blue chair too. So what ones are you planning to recover?. If getting rid of part of it, not understanding. 2 different styles placed next to each other won't look right. I wouldn't like old blue fabrics next to new fabrics or next to 2 different styles of furniture. New 2 seater and old blue chair next to each other, not good :)

on the landscape plan - The road frontage is the left hand curved side & the black rectangle is the position of the outdoor fire.

I am planning to add scatter cushions and think if I do a pair of larger ones for each end so they go over the arms to creat height at each end, and a couple of smaller ones, I want to bring some colour in too. I do like the idea of a larger side light at either end to, Switching the sofas is going to be my final fix if all else fails!!