Start here:
http://www.ordnancesu...uk/oswebsite/getamap/
Click 'Get-a-map'
Enter the search term as SD701820
That will bring up the latest Ordnance Survey map of the area around Green Hill, showing a trig point at 628m
Change the grid reference to SD687793. You'll now see that the trig point at Gragareth is marked as 627m. (You'll also see that it's only just inside Lancashire. The county boundary is shown, running roughly north to south, just to the east of the trig point).
So the Ordnance Survey seems to be satisfied that 'Green Hill' is the answer you're seeking. There can't be any more authoritative source, can there?
Chris