From the airline's website...
http://www.airnewzeal...gage-services-offices
"If you have misplaced an item either onboard one of our flights or in one of our lounges, please contact our local Baggage Services office to report the details of your loss. Every effort will be made to locate this item by conducting a trace. If required, a letter can be provided to support a claim on your personal insurance policy. Found items are held for a period of one month.
Any items found in the airport terminal will be passed to the local Airport Authority.
Please note: Customers uplifting found items from Baggage Services may be required to produce suitable identification."
Rather stupidly, the link to locate the airport's Baggage Service Office doesn't work. It keeps linking back to the same page. I would suggest you make a quick call to the airline's London office on 020 8600 7600 and find out who their handling agent at Heathrow is and on what number they can be contacted.