How is "Interest from UK banks, building societies and securities etc" calculated?

I have completed my tax return online via Government Gateway. The tax calculation shows 'Interest from UK banks, building societies and securities etc: £13,912.00' I'm not clear how this is calculated. I would have expected the total to be based on Box 1 and Box 2 (taxed and untaxed interest respectively) but there seems to be something else going on. My Box 1 and Box 2 figures are as follows: Taxed UK interest (box 1): £8,230.50 Untaxed UK interest etc (box 2): £3,625.95 I am not clear where the figure of £13,912.00 comes from, and cannot see a breakdown of it. I will be paying tax at 40% on this interest, so would expect to be paying 40% on the untaxed interest, and a further 20% on the taxed interest, after the £500 allowance. I have got some dividend income but would not expect this to affect the interest total. I have not got any gilts.

Hi,
We cannot view your personal records on the forum. Please contact our team for advice on your query:
Self Assessment: general enquiries
Thank you.

Not HMRC...the £8230.50 is net, gross it up at 20% (thats the amount at source) = £10288 plus £3625 = £13913 (theyve rounded down a £), thats the amount liable as gross, les any exempt amount, less the tax paid, difference due at whatever marginal rates.