Adam, Charlie - thanks.

I know the issues with cement / concrete / mortar. As water proof as a
slice of toast. My last place had a homemade pond / waterfall which
leaked like a sieve. I eventually left it alone / dry.
This is why I want to get this one right.

The logs mentioned are just for incests / to fill the space. Not load
baring at all.
Again, thanks for the tips. Now I'm getting excited about it.....

One (one of many) questions that has popped into my head now is where
pond 1 goes into stream. I saw special glue to bond PVC liners. I
assume you can get the same stuff for the butyl rubber. Even a large
overlap, capillary action would suck it dry.

Think I'll go re-read everything tonight. Then, I'll write a book
(Credit to you all!

Cheers,
Crispin