Payments on account for the next tax year are automatically calculated by the self-assessment system if you owe more than £1,000 for the previous year as you will read here
If you don't think that the issue which caused you to be underpaid for the previous year will re-occur in the current tax year, you can call the tax office on the number here
to have the payments on account reduced to a more appropriate level or to £0 if you think that at least 80% of the tax you need to pay for 2013/14 will be collected through your tax codings.
If you are receiving state pension, the tax on that will normally be collected by placing a restriction in the tax coding being operated against an occupational or private pension. You, therefore, need to check your tax codings as and when you receive them. It may be that your occupational or private pension is too small for the tax due on state pension to be collected through your tax coding and if that is the case, you will always owe some tax at the end of the tax year.
A new tax year starts on 6 April. You should call the tax office to have your tax codings reviewed while you are on the phone and to have any changes made that are necessary. You might also ask why you were underpaid by £2,000 for 2012/13 and if that is likely to be the case for 2013/14 based on the codings that have been in use for 2013/14.
I hope this helps but let me know if you have any further questions.