You don't pay NI on the pension. You pay NI on employment income up to state pension age.
EDIT:
OP why don't you sign up to one of their tax accounts so you can see what is exactly going on with your income.
If you have underpaid which is what I think you are suggesting it should be able to be recovered over the year by changing (lowering) your tax code. That is fairly standard practice for such a small amount.
But if you have already paid it then id imagine that you would just continue to now pay at the normal rate through PAYE.
EDIT:
OP why don't you sign up to one of their tax accounts so you can see what is exactly going on with your income.
If you have underpaid which is what I think you are suggesting it should be able to be recovered over the year by changing (lowering) your tax code. That is fairly standard practice for such a small amount.
But if you have already paid it then id imagine that you would just continue to now pay at the normal rate through PAYE.