To add:

Liner leftovers can do wonders when you want to force the water to go where
it doesn't seem to want to go :-)

A big flat rock, and some liner (and small rocks to hold it down) to
"force" the water to go over it can make the difference.

Theo

"Lee Brouillet" schreef in bericht
...
The slate has to be *absolutely* level left-to-right; there should be a
very
slight dip forward, to the pond, rather than level. If it's level
forward/back, water has a tendancy to run backwards under the stone from
the
lip (possible source of a water leak). The itsy-bitsy forward lean of the
rock ensures a good "shear" of the water off the rock lip.

Have fun!

Lee