In this scenario Rachel, I would expect a £500 refund and no requirement to pay the first POA in January but they would need to pay£1500 in July 14. Not sure where you get 3 payments on account though?
The client should have to pay the balance of the tax he owes for the tax year by the following 31st Jan.. so for FY12/13 if there is a balancing payment to be made this has to be paid to HMRC by 31st jan 2014, and also the 1st payment on account for his FY13/14 Tax bill (which hasn't ended yet)
So I look back into previous tax years....2011/12 and there are 3 payments on account made 31st Jan 2012 and 31st July 2012 of say 2,500 each.....................................
Look at the payments made during 2013 - in January and July. These are the ones deducted to arrive at the balancing payments due now.