Hi Nutsy,
What year was the P21 for? You moved in with your partner in August, so you would not be entitled to OPF from then but if you were entitled at any point during the year then you get the credit for the whole year.
Is there an explanation for the underpayment? Is it entirely due to withdrawal of one parent family credit or did you also move job or get another incorrect credit? Or did you not claim a credit in your return which you had claimed during the year? If you like you could post the figures in the P21 and we'll check them out. It's possible that a mistake has been made - check the figures against your final payslip for that year and your P60.
The P21 should say at the bottom how Revenue propose to collect the underpayment. It would be unusual to demand it in a lump sum from a person on not-great salary, more usually it would be collected over a number of years, probably four years by reduction of credits.
Don't worry about December's paycheck, they won't reduce this year's credits this late in the year.
Sybil