Thanks all I have sussed it

On the bottom runner I have two safety catches, one that lifts which
makes the window open wider at the bottom to escape in an event of a
fire, and another that pushes that makes the window open "normal" and is
easier for cleaning on the outside of the window.

Just the window closes, the bottom of the frame pushes on this bit of
plastic so it pushes the bottom hinge down a little bit more, making
sure it clears and re-engages the safety catches.