The Advent Killer by Alastair Gunn (Antonia Hawkins #1)

Christmas is coming. One body at a time.
Three weeks before Christmas: Sunday, one a.m. A woman is drowned in her bathtub.
One week later: Sunday, one a.m. A woman is beaten savagely to death, every bone in her body broken.
Another week brings another victim.
As panic spreads across London, DCI Antonia Hawkins, leading her first murder investigation, must stop a cold, careful killer whose twisted motives can only be guessed at, before the next body is found. On Sunday.
When the clock strikes one . . .