Thanks guys for the info and advice. I think we've settled on the concept that the clay is safe for use. So the question is, how to use it.
1 - Mixed in to the soil base.
2 - Placed as a layer on the soil base
3 - Mixed into the capping medium.
4 - Used as the capping medium
5 - A mix of 1, 2 & 3.
As per usual, I welcome your advice, please accompany it with some good reasoning.