Using the first coordinates in the order "Eastings" then "Northings", (or "Along the corridor and up the stairs") we can find the two squares containing the church and Castle X. Now we zoom into that first set of coordinates [Figure 2 on the right]:
The church ("AB 016974"), is somewhere in the RED square, while Castle X ("AB 982035"), is somewhere in the BLUE square. The trouble is that each square is 1000 metres by 1000 metres. So let's get closer.
