I missed the line saying "minus 7.5% tax treated as paid on dividends from UK companies (not repayable) £450.00"
It seems SelfTax gets the calculation wrong though because it treats the non repayable amount as repayable.
Actually, SelfTax seems to be right. I'm getting the same result with ABC SA.
It's interesting though. The notional 7.5% is given on the full amount of dividends and because of the 5000 dividend allowance not all dividend income may result in tax. So if you have a non-resident with some earned income plus let's say £6000 in UK dividends, the income tax is decreased by £450 (£6000 * 7.5%), yet only £75 of tax was due on the dividends (7.5% of 6000-1000). So while it's not repayable, it can decrease the tax liability if there's other taxable income.
I guess this works as designed but still seems a bit surprising.