This proved quite hard to explain. Chris G and Chris B also from Moorfield Juniors had a good go:

In the end we worked out the diagonals=the target number (100) because each across does:

-4,+9,+5

and each down is:

+2,-8,+15

Can you explain it more clearly or add anything to this? If you think you can then email us at primary@maths.cam.ac.uk.