Property company Derwent London said development activity was accelerating as it reported record new lettings worth 21.8m pounds a year for 2013.Derwent posted pre-tax profits of £57.9m for the year to December 31st, up 10.1% as its net asset value rose to £2.37bn from £1.92bn a year ago.Chief Executive Officer John Burns said he expected Derwent's rental values to rise 5%-7% this year and overall property yields to remain stable.He added that demand for Derwent's mid-market office space was "strong and investor appetite for London's commercial real estate remains high"."While we continue to look ahead for any changes to the current favourable market environment, we are moving ahead with our expanding development pipeline," Burns said."Our portfolio contains a wealth of future opportunities supported by a strong financial base. Together these give us the capacity and flexibility to implement our plans with confidence."The company declared a dividend of 36.5p up from 33.7p.Derwent shares were down 24p 2742p at 12.33 Thursday.FP