Overcharging Student Loan on Partnership Income

I'm a bit confused, I've prepared my tax return to cover a small amount of income which I receive from a partnership (£678 this year) and the calculation is saying that I owe £420 (£97 of tax and £324 of student loan) As far as I can tell this should be subject to the following: Income Tax @20% (income from my old PAYE job and my current PAYE job + bonus + partnership income is

Hi SGray,
You would need to submit the return and then contact us direct to view what the full information is on the return to clarify any charges due. telephone 0300 200 3310 and you need to wait 72 hours after submitting before we can view your return.
Thank you.

Thanks, I figured out that my employer had not deducted student loans when I first started and this was the cause of the issue, now rectified via tax return.