Decca Lat and long are usually to OSGB36 datum, change the datum on your GPS set to OSGB36 and input the numbers. Then change the set datum back to the default WGS84. The positions will be automatically put in the right place.
It is the model of the ocean surface that the gps uses. Whith the tide coming in and out there is too much information to program a different height for each point on the ocean every second so a mean height is decided on in order to cut down on the data stored and calcoulations that need to be done. That is why the altitude shown on your gps is normally way out. GPS uses the wgs 84 datum and EGS96 ellipsoid model.
If you use the system I suggested the electronics take care of the different ellipsoid models for you so you can let the machine take care of it for you.