The second "payment on account" of tax for the 2012/13 tax year is due for payment on or before 31 July. However, it is possible to reduce this liability if you think income taxable for that year is less than in the prior year e.g. circumstances have changed, taxable profits are going to be less, there was considerable capital expenditure in the period etc.
Be careful not to overcook the reduction though or you could be charged interest and penalties.
You can do this yourself using the HMRC form or of course we can advise if there is a legitimate reason for reducing the payment and do it for you.