D Leeds

To clarify, I can see that it's the 'High Income Child Benefit Charge' that increases by £40 when I add the interest. Please could you explain why this could be the case?

The problem is that it isn't. I'm doing exactly this, except when I enter £220 of interest, my amount owed in the final calculation increases by £40. I'm a higher tax payer and under the PSA threshold. Something is not working.

I have the same issue as OP. I am declaring interest of £220 in the untaxed box which should be within my personal allowance as a tax payer. However, this makes a difference of around £40 to my tax calculation due. The descriptions on the boxes on the UK Interest page are really unhelpful, as they make no reference to the PSA and give you no indication of whether or not interest under £1000 (or £500 for higher rate) should be entered in the box.