National Grid said on Tuesday that it has commenced a new irrevocable and non-discretionary programme to purchase ordinary shares in the capital of the company between 7 July 2015 and 28 July, as part of its management dilution resulting from the take-up of its scrip dividend.The buyback programme will be managed by a third party, which makes its trading decisions independently of, and uninfluenced by National Grid and is capable of continuing during close periods. The purchased shares will be held as treasury shares.Any purchases will be effected within certain pre-set parameters and in accordance with both the company's general authority to purchase shares granted by its shareholders at the 2014 Annual General Meeting and Chapter 12 of the Listing Rules.