"..isn't it a requirement of the Direct Debit system that any change in the rate must be positively notified to the account holder?"In theory, yes, but in practice it can often be overlooked, which is one of the reasons I try and avoid direct debits. To pay council tax instalments, it is easy enough to set up monthly payments on-line, either each month or twelve monthly payments all set up at the same time.
Richard Greenhough ● 2031d