Having been offered some carpet FOC I decided to go with carpet and sand!
Not a lot of sand, just enough to smooth the contours under the carpet.
The carpet was just about the perfect size.
The liner was quite a bit larger than the hole required. I suppose I could have dug a bigger pond.
Instead I made a last minute bird bath.
All that is left to do now is some landscaping, and then the pond can be populated with plants - I saw a midge laying eggs while the water level was rising!