Jasper,
I live in Kent (Bromley) and am trying to envisage whereabouts there is a pond such as you describe. I thought I knew most ponds of any size in the area, but of course, if its on private land and has never been fished, prospects are that I may nothave seen it.
Is the pond spring fed orcreated by damming a small stream, or just a big hole in the ground?
If its dammed, it will probably have a natural head of fish, and as Richard says, fish have other methods of colonising water.