Looks like you tax affairs are not in a good place, but better to discuss with HMRC to discover the reason.
extract from
http://www.hmrc.gov.uk/helpsheets/e12.pdf
K codes
Tax codes starting with a K have a regulatory limit of 50 per cent of an employee’s gross pay in the pay period. This limit restricts
the amount of tax that is taken from the employee’s pay to make sure that they retain a certain amount of take home pay.
K Codes
Normally, the employee will be allocated their personal allowances by HMRC, and this will result in the issue of a suffix tax code, described above. This will allow the employee to enjoy a free pay amount every time they are paid, therefore, they pay tax on less money than they actually earn.
The simple explanation of a K code is that HMRC have allocated personal allowances to the employee, however, the adjustments they have made have resulted in negative allowances. For example, an employee is entitled to the standard personal allowance but had a company car in the previous tax year with a taxable benefit in the sum of £8,000. The tax code is calculated by HMRC as follows:
•Add Personal Allowances £7,475.00
•Less taxable benefits £8,000.00 equals
•Resultant allowances -£525.00
The effect of this calculation is that the person is due to pay tax on more money than their personal allowances. The -£525.00 will be rounded down to a tax number of 51 and a prefix K placed in front, giving tax code K51. This is then converted to a monetary value in the same way as above, i.e. K51 becomes £519. This monetary value represents the annual amount that must be added to taxable pay. For example
An employee has a tax code K51. To calculate what this means in terms of the amount that has to be added to taxable pay:
•Remove the suffix letter – i.e. K51 tax code becomes 51
•Add a 9 to the end – i.e. 51 becomes 519
•Convert to a cash value – i.e. 519 becomes £519.00
£519 represents the amount of pay is added to taxable pay over the course of the tax year. This is called ‘additional pay.
As with a suffix code, the employee is not taxed on the whole of the additional pay one go. It is allowed against the wage or salary in equal instalments throughout the tax year. For example, using the above tax code and assuming that the employee is monthly paid, the employee will be taxed on an additional £43.25 per month (£519.00 / 12). So, if earnings in the month were £1,000.00, the employee would pay tax on £1,043.25.