Soldato
Slightly extreme but knock the chimney breast out?
edit - damn it, wot joelk2 said (maybe without mounting a portable sun under it though, the fusion is a pita to get out the carpet)
^I like the idea but imagine it would get pretty annoying after a while to keep having to pull it in and out. Especially as if pivoting you might struggle to get it level/consistent position each time, so you'd go and sit down on the sofa and be like 'FFS it isn't level I've got to get up again!!'
As mentioned by a doorway it is going to be a bit awkward if on an arm stuck out all the time.
To be honest I'm struggling to think of a good layout given the fireplace is out of bounds, my initial thought was on a stand in the window bay but I can't see anywhere for the small sofa to go in that scenario given the lack of wall opposite. Log burner in fireplace means you can't put a sofa in front of it, so I would resign yourself to a 'small' TV in that room and maybe put a bigger TV in another room if available. I think part of the problem is that the room is quite a classic layout but just a bit smaller than usual (or possibly a bigger chimney breast) - I have friends have a similar living room but they have room to put the TV on a stand in the corner between the fireplace and the bay window.
Doesn't look like you use the fireplace so mount all your AV equipment in there either on shelves or a custom unit, run wires up the chimney and through to the TV mounted on the chimney breast.
I'd be inclined to mount it on a swing arm and have it outside alcoves ever so slightly - a thin tv won't stick out too much at all so you shouldn't walk into it?
Horribly lazy image editing
edit - damn it, wot joelk2 said (maybe without mounting a portable sun under it though, the fusion is a pita to get out the carpet)
What is that small wall for, left of the TV? Structural?
I have a TV mounted above the open fire in my lounge. The wall only gets a little warm to the touch, even with the fire burning for hours on end (less than 40°C), and in the 7 or 8 years I've had my TV there, I've had no problems. Those alcoves look ideal for a pair of speakers to hide in too...
A log burner should keep the surrounding area cooler than an open fire, too.
Of course, if you don't like having a TV mounted there for aesthetic reasons, then fair enough.
Here's mine: