Derwent London has sold its freehold interest at 13-17 Fitzroy Street, London W1 for £60m, about 10% less than its valuation at the end of 2008.But the Central London-focused property investor pointed out that the IPD UK Monthly Property Index for West End & Midtown offices had fallen almost 15% in that time.The sale of the eight storey office building to a member of the Arup group of companies reflects a net initial yield of 7%.Derwent said the property is fully let to Arup on a lease expiring 2023 at a rent of £4.46m per annum, subject to five-yearly upward only rent reviews.