I know this is an old thread but still found it useful as I'm planning to start digging a pond after Easter.
I had a pond at my old house which was suburban. It was about 2ft 6in deep 8ft long and 5ft wide (couldn't go any deeper as we hit sandstone). We had a waterfall running into it and a fountain at one end to help aerate the water. We had Koi and other fish in it and it was still full of wildlife. Didn't use any special pumps. Dragonflies, newts, frogs, toads, damsel flies and unfortunately herons loved the pond and we were very sad to leave it. This house has a much smaller garden and I'm building the new pond following the Wildlife Trusts guide lines. It will be about 2ft deep at the deepest, 3ft wide and 6ft long. Not planning to have any fish but it will have a pebble beach and pond plants that will allow wildlife to travel unseen to a large log pile and and a slightly overgrown section of the garden. It should get the sun for about 5-6 hours. a day. Hubby is insisting we have the sound of running water so that will have to be incorporated. Can't wait to get started and really looking forward to sitting by a pond again in the summer.
Like the idea of running rain water from the conservatory roof into the pond but it's too far from the house so I'm planning to install a second water butt. It will mean carrying buckets of water up the garden to top the pond up but the exercise will be good for me.