How can I find out HR at DWP UK , if my own manager doesn't know?

I work for DWP UK. I am handing in my notice but don't have the HR details to send it to.

My manager and TL don't know and payroll said I have to ask my manager. I have looked everywhere and asked various people but no one knows. It's like the secret service.

How can I find out HR at DWP UK , if my own manager doesn't know?
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