Royal mail actually. But please do report it to them asap, while the trail may be slightly warmer, and making it clear that it's already been confirmed by your bank as a double theft of mail and subsequent fraud.
Th RM website's attempts to lead people to web pages other than the contact detail ones could almost have been designed to derange. Even the copy-and-paste of their contact page URL leads to a different page. So I'm copying what is on my screen at the moment at personal.help.royalmail.com/app/contact#category-tree-after .
"Call us 03457 740 740. We're open Monday to Friday, 8am-6pm, Saturday 8am-1pm and Sunday 9am-2pm"
Send a message now [personal.help.royalmail.com
[I've tested this link from my preview. It does seem to lead reliably to a Contact form.]
We aim to reply within 3 working days
IMPORTANT: To complete and submit forms on our website, please make sure you have the following internet browsers:
Chrome 46 or newer, Safari 9.0, Internet Explorer 11/Edge, Firefox 41 or newer.
Alternatively, you can contact us at firstname.lastname@example.org
We aim to answer emails sent to this address within 10 working days."