During the credits, certain letters of people's names are red instead of white - I will indicate via parentheses where the red letter is:
"Location Managers . . . Ben Mangha(m)"
"Production Accountant . . . Dav(i)d Jones"
"Production Manager . . . Claire Hi(l)dred"
"Unit Managers . . . Da(v)id Gunkle"
"Assistant Grip . . . Ow(e)n Charnley"
"Production Buyer . . . Blaanid Madd(r)ell"
"Construction Manager . . . Mark Pain(t)er"
"Dubbing Mixer . . . H(o)ward Bargroff"
"Executive Producer for MASTERPIECE . . . Rebecca Eato(n)"
The red letters spell out MILVERTON, a reference to the Sherlock Holmes Adventure "The Adventure of Charles Augustus Milverton," which is a significant source of inspiration for this episode.