Received an aircrew notice yesterday that with immediate effect 50' will be added to all MDA's. The revised altitude will be called the "Non Precision Approach Decision Altitude". This has come about because,according to the notice,the CAA have pointed out that the figures published by AERAD for non-precision and circling approaches are minimum descent altitudes and not decision altitudes, and our present procedures do little to prevent inadvertent descent below MDA in the event of a go around from a non-precision or the level segment to the MAP on a circling approach.Our SOP's were to fly non-precision approaches as a continuous descent to MDA and to go around at MDA if nothing seen(incidentally until a couple of years ago we used to fly level at the MDA until visual or the MAP).By the way this is at Air 2000.