Derwent London, a developer of Central London offices, said the market continues to strengthen as it submits plans for three new projects in the capital.The group's veteran chief executive John Burns said, "Although economic conditions remain uncertain, we are encouraged by the continued improvement of the central London office market and our letting successes."Central London lettings, excluding short-term transactions, were 7.9% above estimated rental values at 31 December 2009 and 2.6% above 30 June 2010 values.The group said it has signed 35 lettings in the third quarter totalling 149,800 sq ft that will generate rental income of £2.9m per year.Its Angel Building, EC1 was completed in September while the go-ahead has also been given for its mixed-use scheme at 60 Commercial Road, E1.Planning applications have been submitted for three further schemes totalling 605,000 sq ft. The firm said it will continue its acquisition path after buying the Central Cross development on London's Tottenham Court Road for £146m.Net debt increased to £902.0m from £749.2m during the quarter due to acquisitions and investments.