Half-year losses grew at West End-focused office developer Derwent London as the value of its property portfolio tumbled by £259m.The UK firm reported a loss before tax of £223.3m for the six months to 30 June, up from £144.7m in 2008, although underlying recurring pre-tax profit rose 72% to £29.7m. Adjusted net asset value per share dropped to 993p, 19% lower than the year end figure of 1,226p.Derwent's property portfolio, 93% of which is located in central London, was valued at £1.86bn at the end of June.'We have made a strong start to the second half,' said chief executive John Burns. 'I am encouraged by continued evidence of yield stabilisation and the demand we are seeing for our properties as demonstrated again today by the letting of 45 Whitfield Street.'Derwent said Tuesday it has secured a 12,200 sq ft pre-let at 45 Whitfield Street, W1 to Target Media Group, which will take the entire building on a 10-year lease at £345,000 per annum.Burns added: 'Derwent London retains significant financial fire power and this, combined with the flexibility we have to initiate developments for delivery in late 2011 or 2012, means we are well placed to catch the next stage of the cycle and build on our track record of delivering long term value to our shareholders.'The interim dividend stays at 8.15p a share.