When is corporation tax actually due? I'm confused about the deadlines

You're right about the nine months and one day — for most small companies, corporation tax must be paid 9 months and 1 day after the end of your accounting period, and the money has to reach HMRC's account by that date, not just be sent. The filing deadline for the company tax return itself is separate and later: 12 months after the end of the accounting period, so the payment comes first — this catches a lot of people out. If you're late paying, HMRC charges interest from the day after the due date, and late filing of the return brings separate penalties. With a March year end, your payment would typically fall at the start of January the following year, so setting money aside through the year is a sensible habit to build.