Many thanks for the suggestions. I've been away since last Tuesday on a family matter, so the pool has just sat, no pumping. Water loss seems more than the evaporation from a measuring cup left out. Sunday afternoon I ran the pump sourced from the main drain (water level is below the skimmer) for two hours and saw no dectable drop in water level. This is encouraging!! It suggests that the returns are okay.

Today I'm adding water to enable using the skimmer; I'll try the two hour test again. Since the liner is in rotten shape I'd be "happy" to find that a replacement would solve the problem. I'll also continue the bucket test.

If the two hour test is successful I may still pop for a $250 visit from a leak detection company. BTW as far as I know the entire underground plumbing run is under concrete. Ratz!

Here is a link to the two products a Google search uncovered for a leak fixing additive: http://www.fixaleak.com/main.html and
http://www.lo-chlor.com/pool_chemicals/leak_sealer.php

Searching through old threads on leaks in this forum and another one uncovered one responder who'd used such a product in 2005; an email to him has produced no response. If you're out there please respond!