It depends on the type of Direct Debit you have set up, and when you are looking to make the additional payment.
I have a Direct Debit to collect the minimum amount:
If you choose to make an additional payment before the Direct Debit is collected, then your minimum Direct Debit payment will still be made, unless this amount takes your account into credit. In these circumstances, the Direct Debit would be adjusted to ensure the account does not go into credit.
I have a Direct Debit to collect the full amount:
If you make an additional payment before the Direct Debit is collected, then your Direct Debit payment will reduce accordingly, ensuring that overall you only pay the full amount required.
I have a Direct Debit to take a fixed amount:
If you choose to make an additional payment before the Direct Debit is collected, then your fixed Direct Debit payment will still be made, unless this amount takes your account into credit. In these circumstances, the Direct Debit would be adjusted to ensure the account does not go into credit.
Please note that any additional payment made within 3 working days of your Direct Debit date will not affect your Direct Debit amount as the request will already have been sent to your bank.