You are looking for a magnetic micro tube.
The memorial was unveiled in October 1921 by Colonel Joseph Gaskell who had lost his son Frank in the war. It stands on the base of a medieval preaching cross. The cross was destroyed by Parliamentarians during or after the Civil War in the 17th Century but the plinth was left in situ. In 1916 the plinth was used to display a captured German artillery gun!
The cross was unveiled to commemorate the fallen of the First World War. The prayer book was erected under the cross between 1947 and 1948 and dedicated to the fallen of the Second World War.
It may be noted that the dates for the First World War are inscribed 1914 -1919 as opposed to 1914-1918. Armistice was agreed on 11/11/1918 but the Peace Treaty of Versailles was not signed until 28/6/1919.
The given coordinates will take you to the War Memorial in Llantwit Major. Here you will find a cross inscribed on its south side. By using this inscription:
A = Number of letter As
B = Number of letter Ms
C = Number of letter Es
D = Number of letter Ss
E = Number of letter Rs
F = Number of letter Us
The final cache can be found a short walk away at
N 51º 24.AB(E-1) W003º 29.DCA