Unfortunately a number of the Financial institutions continue to behave like this, as they were not required to change their numbers as part of the EU Directive.

With regards to Santander Consumer Finance they do have a complaints line which is both an 084 number and an 034 number.  Perhaps it would be worth registering a complaint about the use of the 084 number to change your details on.

I did call the 0344 version of the number you have listed but it's not in service.

Their contact info is here.
It would appear you can manage your account online as well.