See GPS systems and mapping in 21st Century for details. Try to find out which system was used to provide any positions you are given. There are other geodetic systems but you are unlikly to come across them except possibly WGS72 which is very close to WGS84. N.B. New Admiralty charts will use a new Euopean geodetic system which is, as far as normal users are concerned, identical to WGS84 . . These provide a rather more difficult problem.
Most fishermen tend to use uncorrected positions.
If you are converting between Decca and Lat and Lon you should work on corrected numbers. Some GPS sets such as the French Valsat professional sets have the option of a Decca read out. I think these must also apply the correction automatically because mine always compared very well with the Decca figures, not exact but much closer than the known Decca error. As you can see it all gets a bit involved. If you have a number of positions from the same source, then once you find one note how far out it is and then apply your own correction to the others.