Clay sub-soli. (Presteigne LD8)

I want some sub-soil clay to mix with grantite dust to make a substrate to grow moss in a garden. I need a barrow full or so. I have builders sacks, and happy to pop on site, bag it and remove. Is any one in a hole digging up soil they need to dispose of because it’s generally useless for normal gardening? If so, a few bags of the stuff that you tend to hit about a metre down round here would be great.