As Radrick007 says.
For official information, try the Royal Mint information page about UK pound coin designs and when they were issued.
http://www.royalmint.com/Corporate/facts/coins/OnePoundCoin.aspx
Many counterfeit pound coins are circulating, and its not uncommon to find that counterfeiters have put the wrong date with a design.
Check also the quality of the design, the edges should be sharp and clear, not with the appearance that they have "melted", even allowing for wear in circulation.
Matt