Santander SAR hassle

I sent off a SAR to Santander then got a letter back asking for proof of address which came in the form of 'change of details' I filled it out and sent it back, now received a letter back saying "Sorry we cannot make these changes at the moment because some details are missing"

I now have to send back the form ensuring it's signed with full account number and full details of old and new address with a form of ID, which I sent last time. A driving license was acceptable according to their 'iD they accept" letter.

The above was done on my original SAR form, I signed, put account number, previous and new address and sent the correct form of ID.

Is this the normal way for a SAR? Is it easier to just do it online? Here

https://www.santanderconsumer.co.uk/your-data/access-your-data/subject-access-request/

Thank you.