Wow. Turns out it was my sloppy work all along. Big suprise, huh
2 things I had wrong:
1- the BG layer was set to 50, 50 for parallax which was making it look like the screen was in the wrong spot vertically. I changed it to 50, 100 and then it looked right.
2- I had the ground on the FG layer instead of the action layer. The player sprite looked like it was falling indefinitely because technically it was. I didn't realise that the original layer is set to 'Transparent: no' and so the ground was not showing up when I ran the layout, even though I could see it in the editor...
So thanks for the help everyone - the error was occurring between the chair and the keyboard