Derwent London, which rents out offices in the capital, said it has managed to keep vacancies low and is seeing an increase in investor demand, particularly in the city centre.Only 4% of properties have been vacant, said Derwent, which has reduced debt from £849.3m to £798.1m over the quarter to September 30.Derwent said its Angel Building at Islington and its Arup Phase III project in Fitzrovia are 65% pre-let.The firm said its strategy over the past two years of preserving income and carefully managing capital resources and financial ratios would continue, but that 'recent signs of improvement in the central London market have meant that we are now preparing for the next stage of the property cycle.'