Statutory Maternity Pay (SMP) is paid at 90% of your earnings for the first 6 weeks followed by a flat rate, which is increased by a few pounds every tax year. In 2008/09 the flat rate is £117.18. If you earn below the Lower Earnings Limit, then you are entitled to receive 90% of this as the flat rate.
Statutory Maternity Pay is paid for the first 39 weeks of maternity leave, so the flat rate mentioned above will be payable for 33 weeks unless your employer offers something better than this.