Under the self-assessment system, taxpayers must make payments on account of their tax liability on 31 January during the tax year and 31 July after the end of the tax year. Any balance due is payable by 31 January following the end of the tax year.
The second payment on account for 2009/10 is due to HMRC by 31 July 2010.
To make sure that you don't miss important deadlines, check out the calendar page of the Writetax website.