Santander Bank

18cc

Would you be prepared to share what Santander told you about how the fraudster was able to quote your transactions on your account.

Terry_Towelling

It's sounding very much like someone close to OP who already knew a fair bit, spoke to Santander and, during that impersonation, Santander gave out some other detail in the belief they were talking to the genuine account holder. That extra detail enabled the 'fraudster' to get further into OP's financial affairs.

If Santander acted improperly then, despite the lack of any loss, and, given OP's view on what happened, I am surprised to hear that OP is not baying for Santander's blood. Perhaps they are more intent on 'outing' the suspected perpetrator.