Surely the fact you can turn round in a circle when on top of a mountain with 360 degree view of the horizon like it is prove it is round?

Yes there would. And stop calling me Shirley.

Since I was old enough, I spent a lot of time on the Atlantic Ocean. Seeing boats disappear over the horizon was proof enough for me we live on a round planet.

Flat things don't always have corners, so that isn't proof, but if you go to the shore and watch a very big ship sail past the horizon you'll see the upper part of the ship last.

Actually seeing any lunar eclipse pretty much guarantees the fact that the earth is round.

No.
If the earth was flat you would see no corners.
You would see the same in every direction -- assuming there were no other mountains to block your view.
You would see much the same as you do on a round world.

Except, on a flat world you would be able to see much further - limited only by the atmosphere absorbing the light. On a round wold anything over about 50 km away would be below your horizon -- depending on just high the mountain you are standing on is.

On a flat earth, using a telescope you would see further. But thatwouldn't happen on a round world