Having got out of order on these, now going back to read the first of the Ash Henderson books.
From the Blurb:
The tabloids call him the Birthday Boy. He snatches girls just before their 13th birthdays. One year later the family get a homemade card in the post - 'HAPPY BIRTHDAY!!!' scrawled in blood-red ink above a Polaroid photograph of the missing girl. Detective Constable Ash Henderson is seconded to the investigation.