Exposed gap between brick rows

Either lime mortar or 1:1:6. Lime mortar is 3:1 sand to lime (by volume)

NT

"Stone" window sills in edwardian houses are quite often actually rendered bricks (cement, not lime). Keep an eye out for cracks in the render which can cause the effect you're seeing (and eventually the render will break off by frost action once water gets in). Also check the drip channel on the underneath hasn't been blocked with excess coats of paint, allowing water to run back under the sill. The render coat under the window frame may not be very waterproof (original window frames were often deliberately put in place before the render set in order to make a good seal, but any replacement PVC windows which could be leaking water from the frames might well leak through the render).

the problem its not under the window pane itself but the supporting stone/cement block which was had mortar originally with carbon and ash to the brick bellow removing this layer has exposed a gap of 3cm all the way round the bay - hoping that the window is still supported by the two ends - will render as advised and seal properly checking that channel as you have advised - thanks