Clay pond help please

I've recently scooped out an old silted up pond in my garden which filled up
and overflowed every winter but dried out in the summer. It's a fairly
large triangle of about 20 yards each side. It takes four days to fill with
a hose and then two days to drain. The clay base (of unknown thickness)
seems watertight, but I reckon it's leeching out where the clay meets the
topsoil sides.

What are my options? It's a bit big for a liner but I understand that the
clay could be extended up the sides. What is the technique for doing this?
How thick does the layer of clay need to be? How do I test that the clay to
be used is up to the job?

Many thanks in anticipation