we are down to the clay and would like some advise on what to do next
Filling in a Clay Based Pond
Certified GKH Gardening Expert
If the ground is still holding water, you will need to pump it out.
Then you can fill the pond hole in with fill or other top soil.
If the pond area is quite large and you have water drainage issues in the area around the pond, you may need to have drainage tile installed.
Filling the hole with rubble would not be recommended.