Anyone who pays tax on a self assessment basis - the rich, entrepreneurs or those with abnormal financial affairs - know that they have to pay tax on account. For the tax year 2009/10 as an example, they'll make payments on 31 January and 31 July 2011, with a balancing payment on 31 January 2012 once the tax return has been accepted by HMRC.
To aid prompt payment, HMRC sends out statements to taxpayers telling them of their liabilities. However, it appears that HMRC hasn't ordered enough paper for these statements because more people than normal require a reminder. As result the statements will be going out in August - later than the 31 July payment deadline. To allow for this, so long as payment is made within 30 days of the statement being sent, no interest or penalty will be charged.
A little common sense that if pushed to the wire can help earn a little more interest on HMRCs money.
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