Section 419 Assessments

We received Notices of correction to Corporation Tax returns last week for two different clients, where HMRC have revised the Self assessment to show Section 419 tax where Directors Loan accounts are overdrawn.  Although Corporation Tax is not yet payable, HMRC are adjusting the tax calculation on the basis that relief is "not available on the basis of a proposed repayment at some time in the future".  I can't quite see where they are coming from...nor why they are still referring to Section 419 instead of Section 455...