The creation of a pond by machinery is considered an engineering operation under the Town and Country Planning Act 1990 (as amended). Even if your proposed pond will be small and shallow, the excavation of a pond may be defined as an engineering operation which may require planning permission. Its always best to check with your local planning authority before hand.
This guidance is taken from The Freshwater Habitats Trust website, which is a great source of information for creating and managing ponds.