HMRC dos not reconise the return earned to be interest for tax purposes.
That if the return has the nature of income, then it will likely be within the scope of miscelanaeous income.
As such, you may apply the £1000.00 trading and Misc income allowance.
BIM86000 - Trading and Miscellaneous Income Allowance (TMIA)
Tax-free allowances on property and trading income
If misc income is less than the £1000.00, then you need not declare the income, if more than £1,000 you can deduct the allowance and then declares the balance.
Entry for Misc income is at Box 17 of the main Self Assessment return.