how to repair leaking window frame

Hi Looked through previous threads and seems could be a few reasons, but wanted some simply explained bespoke advice on this please Leak coming into top of window when rains heavy (see red circle picture of entry) 2 bed semi property, downstairs kitchen window Can see on pics, and not sure...

During a sudden exceptionally heavy and, luckily short, rainstorm this morning, water came pouring in at the top of my window frame from where the frame meets the masonry. Outside, the wall is covered with that thick render material which looks like tapioca. (Sorry I don't know the name of this...