You have all been most helpful!

I've been thinking about this on my lunchtime stroll to the shops.

I think I will build a box the rough height and width I want out of
bricks. This allows me to fill the box with logs. A space to be used
by critters at some point I hope.

I can then shape around this with concrete. Once the rough shape is
done, I can put down some of the underlay, some thick liner (I found
1mm rubber one) and then another layer of underlay, then finally more
concrete to hold the rocks in place. Enough [real] rocks and pebbles
and you will not see the main structure.

What about concrete cracking? Would the fact that the liner is between
two sheets of underlay resist tearing? Small cracks I would imagine
would survive. Greater than 1mm I have a bigger problem.

This will be the waterfall as well as the cascading stream down to
pond 1. I can then have little stream to pond 2. Finally, I like the
idea of the 3rd pond. Very neat. It'll have to be = pipe volume +
Stream volume + Waterfall volume.

Thanks for the links and info. Plenty reading for me to do

In my mind's eye, I can see this clearly. Now to implement this!
Anyone got any spare time? I have many spare beers (Well, not spare
but willing to share) and lots of food. I'll provide everything, you
provide help / expertise

Cheers,
Crispin