...I realised if I was out on a pinnacle in the middle of the ocean at height and no land was visible, and I turned 360 degrees how could there possibly be any curve from left to right it would have to be straight whether it was round or flat earth. If I looked at the horizon and it really was curved every time you turned slightly there would be a hump in the horizon so to see 360 degrees you would need a series of arcs joined together.
I see my point has been missed, perhaps I didn't make it clearly enough...
In your above scenario, how far could you see in any given direction? Is it a pretty much constant distance; ie, you're in the middle of a circle of visible area basically? Assuming that's the case, picture this (thought experiment): imagine you are shrunk down to 1mm tall, and stood on top of a very large beach ball. Turn 360° and the 'horizon' is going to be the same height wherever you look (and the same distance away), just like on earth (the same could be achieved by standing on a record, but I'm a roundy, so beach ball it is

). Now get a normal sized person to place a long straight edge against the ball so that it touches somewhere along your horizon. What would that look like from your 1mm tall perspective? There'd be gaps under the left and right ends of the straight edge, wouldn't there? Have I made my point now?