[julie_m]

There should be a baffle inside the kettle lid, restricting that vent hole even further. A small amount of steam will always escape from it but the idea is, under normal circumstances, it should be easier for the pressure to displace water via the kettle spout than to eject steam from the vent hole. But if the spout is blocked, the vent hole provides a way for the pressure to escape.

Could the kettle spout be partly blocked by limescale? My water supply comes from the Peak District, so you can imagine what it's like .....