SLC refuses to refund my overpayments - are they right?

MagicByNature

I am on plan 1 loan. Midway through 21/22 tax year I started a job, and student loan was collected automatically through PAYE - £694 in total. In that tax year, I made £19353, and the threshold was £19895. However, I also made £3440 from self-employment. I therefore made a total of £22793, minus £19895 that gives us £2898, of which 9% is £260.82 - which I believe is what I should have paid, and I therefore should be refunded £433.18.

However, SLC says that's wrong, and I won't get any refund. 

Who's right?