http://www.uktaxcalculators.co.uk/Use this above
Bear in mind that if this is for year end 2012 then you will need to pay some in January and some in July £720 each with balancing payment
£1440 in January if this is your first payment. (this covers April 2011-March 2012)
If it is then the below is also needed to be paid
Another half if this is your first year 'on account' (which covers April 2012 - Sept 2012) Then the other half in July (this is the other half 'on account Oct 2012- March 2013)