On 1st April 2015, HMRC updated their system to change the way they allocate repayments of overpaid tax. 

HMRC will only process the refund using the details entered in

HMRC Forms:

SA100 Core Return > Page 6 > boxes 4-14 

SimpleStep:

Other information > Bank Details

If there had not been any payment made by debit or credit card prior to this. Therefore, your request will be ignored and the repayment will be automatically issued to the card they hold on file for the payment made. 

This is supported by the SA150 notes (How to fill in your tax return) on page TRG14 (extract below):

If you have paid too much tax

If you paid your tax by credit or debit card, we will always try to repay back to your card first before making any repayment as requested by you in boxes 4 to 14.

Further to this, HMRC have provided the below criteria which must apply before they will apply this rule:

1. The most recent payment was made by card; and
2. The payment was made after the implementation of the new process (from the 6th April onwards); and
3. The repayment due is no more than the amount of the last payment received; and
4. The payment must have been made within the last nine months.

Unfortunately, there is no way of overriding this as it is automated by HMRC's system.