Having read TerrorTome, it's really obvious that Matthew Holness knows a thing or two about horror, the first of the three interconnected "horror" tales is obviously Garth Marenghi shamelessly ripping off Clive Barker's Hellraiser mythos to his own ends, especially with the character who is definitely Not Pinhead, but rather "Type-Face, Dark Lord of the Prolix", who when he is introduced is described as what looks like pins studding his head, but as the narration takes pains to emphasize are NOT PINS, not pins at all but typewriter keys.
There's a lot of parodying of the basic horror stylings of your Shaun Hutsons, your Guy N. Smiths, as well as any number of pulpy horror writers who at least had a couple of slim paperbacks published in the 1970s and 80s by Sphere or New English Library or Hamlyn and so on inhabiting spinner racks. The middle story, Bride of Bone, has a Hutson-ish feel to it, with its story of a serial killer/mad doctor building an army of living skeletons from necrotized bone (referring to them as his "Boners") and the third story continues in this vein, but also parodies the basic idea of King's "The Dark Half". Main character Nick Steen, tough-as-nails, uncompromising horror author (and obvious stand-in for Marenghi) has to deal with more of his creations brought to life, including the dark twin of Steen's own fictional stand-in hero, horror author Gareth Merang. Turns out this ‘dark fraction’ has his own dark twin, and that dark twin has his own dark twin, and so on and so forth until thousands of alternate versions of Steen are on the rampage.
The second volume of interconnected tales, Incarcerat which, after an attempt at an Altered States-style mindscrew, comes "Arabella Mathers", an overblown Gothic melodrama with a dash of "V.C. Andrews", the story of a cranky horror author hoping the new book he's working on will make Gothic style horror popular again (and earn him enough money to get his ex-wife to lay off and get her to take sole custody of their 12 year old daughter), while working on said book in a creepy old Gothic style manse. (Though a lot of the black humor from this segment comes from the not-even-subtext that Garth Marenghi has a lot of weird and self-serving ideas of what it is to be a parent). Then we get the tale of the "Randyman", star of a whole series of Steen's novels, a Freddy Kreuger-meets-Candyman tale of a restroom attendant at a public park, falsely accused of being the local flasher exposing himself to youths and then getting killed by a toilet-based prank arranged by some of those youths. Now he has become a dream-stalking demonic flasher, who the legends say can be summoned if you say his name...seventeen times.