(Adds detail on planned index-linked sale in Nov) LONDON, Aug 31 (Reuters) - Britain’s debt office said it planned to hold 20 gilt auctions in the October-December period plus two syndicated sales of British government bonds, one of which would be for a new green gilt with a 20-30 year maturity in mid to late October. The Debt Management Office also said it planned to sell via syndication an existing or new index-linked gilt with a maturity of around 40 years or longer in the week starting Nov. 22. (Writing by William Schomberg; Editing by Andrew Heavens and Alison Williams) Our Standards: The Thomson Reuters Trust Principles.