Draught through dead light window

[misterboumsong]

I have a fixed dead light window in my cupboard which seems oddly to be very draughty I looked outside and wonder should the gap between the windows and the cull be filled is the only area I can suggest that would allow air in see pic

 

[crank39]

The gap is meant to be there, it lets the water out as part of the concealed drainage system in the window