LONDON, Sept 21 (Reuters) - British public sector net borrowing, excluding state-controlled banks, totalled 20.5 billion pounds ($28.02 billion) in August, the Office for National Statistics said on Tuesday. Economists polled by Reuters had on average forecast borrowing of 15.6 billion pounds for August. ($1 = 0.7317 pounds) (Reporting by David Milliken and Andy Bruce)