Schedule 234A - penalty despite tax paid?

A taxpayer is questioning why they've incurred an additional £6,000 penalty under Schedule 234A for three months (Oct-Dec) despite having paid their £200,000 self-assessment tax in September and already accounting for a 1% monthly interest charge for two prior months. They believed the tax liability was settled and are confused by the increased demand, especially given the extended ITR filing deadline in December. Another user explained that the interest under 234A is calculated from the original ITR due date until the actual filing date, suggesting the taxpayer should have filed sooner after making the payment.