In reply to Chopper:
The difference between datums varies depending on where you are in the world. Around here WGS84 puts you about 180m NW of the same position in ED50 (or vice versa, I can't remember). So it is entirely possible you are on different datums and they match at your alternative location. Having spent hours out on my boat trying to find shipwrecks with position numbers given to me in unknown datum I've learnt to input the position with my plotter set to WGS84, then again at ED50 and search both locations. For a given location, setting the datum to WGS84 then a.n.other datum will give two different sets of lats&longs. WGS84 is the modern standard so I always use that. With reference to the position you wrote out for us, not only are there different datums, but also different grids. You have 'OS grid' selected in your options rather than lat&long. I've not used Memorymap, but my garmin GPSes have options like OS grid, loads of other national grids then lat&long in multiple formats - degrees-minutes-secs-decimal secs; degrees-decimal degrees etc. I always use degrees-minutes-decimal minutes (thousandths) in the boat to get a position like 49-56-405 2-27-959
Post edited at 23:43