In reply to Russell Lovett:
Sorry don't know how to enter on that watch, but is that not just a 10 figure grid ref that your watch already has? It seems correct for Great Gable as a grid ref. If so can you just add the 10 figure for the Ben assuming you know how to do that? Maybe that is what your asking! If so download the manual for your particular watch would be the easiest way.
The 10 figure coordinates for the Ben could be NN1667671283 if you believe
http://www.haroldstreet.org.uk/waypoints/download/?hillnumber=278&/Ben+... Better to check first for your own safety!