Folk go OTT about fixings.
Unless you've got a false chimney breast constructed from non-combustible materials (e.g. metal frame + fireboard/vermiculite to meet Part J of Building Regs) then chances are you're going to be fixing in to brick or block or stone of some description. Bricks are 100~110mm deep. Concrete block and aerated block is 100mm deep. This means there's at least 100mm of some kind of solid building material (brick or block) separating you and your drill bit from the chimney liner.
On top of the brick/block there's the surface finish. It could be plaster or dot-n-dab. Allowing for the thickness of the top surface finish, all you need is a
maximum of 50mm penetration in to the brick/block for a fixing. Fixed the right way, those little red wall plugs that are 30mm long will hold a staggering 50~100 kg a piece. "Fixing right" means getting the correct diameter hole drilled (5.5~6mm masonry bit) and no more than 40mm penetration of the brick/block. Ensuring the fixing isn't in the mortar bed between bricks or at the edge of a brick -
that's why you use multiple fixing points - and finished with the correct size screw diameter/gauge (3.5/4.5mm - No. 6/8). Fix your bracket with 8 screws and even if you only get four solid fixings you'll still have 200 kg of load capacity. You could hold two chubby blokes off that. A telly will be no problem.
Here's a guy testing red wall plugs
https://www.youtube.com/watch?v=0Lx7uaboPdw
Someone would have to be a complete knucklehead to drill all the way through the block work and damage the liner. Use some tape on the drill bit as a depth marker.
If there is a problem with a telly over a log burner then it's dust. LCD screens aren't sealed. The convected air will carry dist and smoke particles in to and over the inside layers of the LCD screen.
Heat is a secondary issue. A mantle can deflect a bit of it. It just depends really on how hard the fire is driven.
