NOW I know!

I was sure the pond had a leak.
I stopped the pump ... No water loss.
Switched pump on ... Water loss
Checked old delivery system ... moisture around junction + damp soil = leak.
Renewed delivery connection + "Urn".
Leak vastly reduced.

NOW I know why it didn't totally stop.
Well! Bath night + a bit of evaporation.

The up-side ... I don't need to water "The Heap".