I dug the hole out with machine, it rained 2 days later and as we are had wetest June & July in history (officially) the pond is not draining to allow me to get on with it.
I have pumped it out 5 times so far ... today it was 3/4 full again.
Exactly. And once you get the liner in the water pressure against the clay will pack it even tighter. Basically a clay bowl like on a potter's wheel.
I wonder that maybe I could create a small hopper under one edge stone, that allows water to enter when pool is 'over filling' and have a float valve operated pump in that (hence my comment about a float operated bilge pump )
Or even just have a 2" pipe 'hidden' under an edging stone sloping down to a 'created’ hopper outside of pool .. and put bilge pump in that ....
As long as the pump can keep up. You could do some calculations on rain fall in your area to see.
For the pipe under the perimeter stone you'd really want the pipe to be thru the liner, not just sitting on top. This would be called the over flow pipe. Normally they go thru the side underwater and then 90 up to the surface. But thru the side above the normal water line is fine too, but thru the liner.
I could build up edges so pool is 6" or so above ground level, but no higher or it would not look right. This would prevent any run in of water .. but not rain water of course.
Actually this is exactly how the floating liner issue is normally solved. If your yard is truly flat and you build up the pond edges by 6" you can install a normal overflow pipe and just let it drain into the yard. Basically what you have in your picture minus the sump pump. The entire yard, your entire neighborhood would have to be covered in 6" of water before your liner would float. Even 2" can be enough if you're precise on the build. Of course the higher you go the safer.
This can be a win win. One issue with a pond build is what to do with the soil. If you use some of it to build up 6" around the pond and slope it away maybe 10' you'll hardy notice that it's built up. Once rocks and plants are added you won't be able to tell at all.
One thing you may want to consider is a concrete collar around the edge of the pond. That keeps the edges from caving in which makes it easier for water to get behind the liner. Also it make a precise build a lot easier so you know your edge is level and whatever distance you want above grade. Then you can add the soil and slope away from that collar. The collar can be as simple as just blocks, or a step up and use bond beam blocks with rebar and filled with concrete, or top grade poured concrete with rebar. I've just used plain block many time and it works very well. Fill the blocks with clay and tamp down.