When the minimum wage came into force in 1999, the adult rate was £3.60. Five years later in 2004 it had increased to £4.85. Another five years on in 2009 it stood at £5.80. Not sure what the exact figure is today but I would hazard a guess at about £6.50 to £6.70 meaning that every five years it roughly increases by a measly £1.
But whether £9 p/h will become the minimum living wage in 2020 is anybody's guess.