Here's a bit of an explination from a site.
nicker = a pound (�1) Not pluralised for a number of pounds, eg., 'It only cost me twenty nicker..' From the early 1900's, London slang, precise origin unknown. Possibly connected to the use of nickel in the minting of coins, and to the American slang use of nickel to mean a $5 dollar note, which at the late 1800's was valued not far from a pound. In the US a nickel is more commonly a five cent coin. A nicker bit is a one pound coin, and London cockney rhyming slang uses the expression 'nicker bits' to describe a case of diarrhoea.