Plan 1 loans: Are any repayments I have made direct to SLC taken off my self assessment bill?
No. You can make voluntary repayments direct to SLC at any time. But if you do so, these payments will not reduce the amount that you have to pay under self assessment and should not be included on your tax return.
If, however, you are employed and repayments have been deducted under PAYE by your employer, these do come off the amount due under self assessment.