Roy , the task was to find an equilateral triangular stone in a monument that is revetted and walled with multiple stones , in an area where angular and triangular shaped stones appear to be quite common .
Angular stones are probably much more common and can appear triangular when only partially exposed , do you have a pic of the stone fully exposed to allow for an assessment of it's triangularity ? If when fully exposed it is actually is an equilateral triangle , then how many others are there under the turf ?

As the dowser seemed happy to be put to the test would she consider attempting the same problem with a buried stone/wooden triangle in an area more "stone free" or anything foreign that could be located by the same method . I would be more than impressed by a dowser, as long as it wasn't Derren Brown , finding a foreign object buried in area the size of KAH .

The female burial could be a test but then again the rectangular monuments in Brittany have burials in their corners andf if by some chance KAH was related that is where you would expect to find a burial deposit . And then again there might be no deposit .