If your self-employed accounts are for the calendar year, you will be preparing accounts to show your turnover to include all amounts invoiced in the particular year regardless of when paid.
Therefore you will pay tax on the income in the year that the accounts are assessed.
e.g. if you prepare accounts for the year ended 31 December 2008, and you invoiced for this work in 2008, the taxable profit arsisng from the 2008 accounts is taxable in 2008.