I decided, on the spur of the moment, to make a pond. DH agreed to help. He's got a jcb type digger, so very easy.
We lined the hole with HD polythene and started filling it with tap water. As the weight of the water increased, a tree root that we hadn't noticed poked through the polythene and made a hole.
I baled out the remaining water that hadn't leaked away. I took some time rescuing worms/spiders/slugs that had fallen in.
DH suggested lining it first with some thick felted polythene that we had lying around after an old roof had been removed from the house. We used that, and put the polythene over it.
We refilled it and it has held water this time, no leaks. We've added a wooden pallet temporarily to help anyone falling in to get out. We're going to the garden centre to get some plants for it.
I'm a bit concerned though that we used tap water, because I've read it can be harmful to some creatures. However, the advice was that the chlorine dissipates after 24 hours, so it should be okay.