Update:
I’m now getting the board x,y values, although still not totally accurate. In some places I might get x=2, despite being about 4 or 5 pixels inside column 1.
I’m not sure I can count on each square (of the image material) being exactly the same width or height right across the board, even though by eye it appears uniform. I might have to do some pixel colour interrogation if we are close-ish to a border of 2 squares and make an intelligent guess or perhaps just deny the click if it is within say about 5% of the border.
So thank you again Everynone, I’m pretty happy to get this close. 
Cheers,
Max