Be sure to properly identify which species of newt you have, using the identification pages on the Amphibian and Reptile Conservation Trusts web site. Newts can be hard to identify and can have features (like a temporary breeding crest) that occur in more than one species.
Great crested newts are strictly protected in the UK. If you do have great crested newts in your pond you will need to be aware of how this law affects your management of the pond. Please talk to Natural Resources Wales or Amphibian and Reptile Conservation Trust.
After receiving information from you statutory agency you should inform your local Amphibian and Reptile Group and the local Biological Records Centre of the sighting. These records help these groups understand the distribution of this species locally.