By Amber Murray GPE, one of central London’s biggest listed landlords said that the capital’s favourable investment market, with improving economic conditions and falling interest rates, has allowed the firm to push ahead with its expansion plans. Great Portland Estate signed 15 new leases and renewals during the quarter to 30 September, generating annual rent of £6.1m, with market lettings on average 6.4 per cent ahead of last year’s estimated rent value (ERV). Following the 22,500 sq ft letting to TK Maxx the firm announced last year, in the quarter GPE completed three new retail deals at Mo…