To avoid paying late you must make sure HMRC have cleared funds by the due date. If you pay electronically this is the 22nd. However when the 22nd falls on a non banking day (weekend or bank holidays) HMRC must have cleared funds by the last bank working day before the 22nd.
For example: 2010-2011 PAYE month 9 for period ended 5 January 2011.
The due date of 22 January 2011 falls on a Saturday (non banking day), so if you are paying electronically your PAYE payment must clear HMRC’s bank account by Friday 21 January 2011. You may therefore need to change your payment date to ensure you pay on time. If you do not do so, you will be noted as paying late, and late payments for this tax year may result in a penalty being charged.
For more information about payment deadlines