[…] the dread Of dying, and being dead, Flashes afresh to hold and horrify. (8-10)
Make no mistake, this speaker is spooked by death. Larkin's word choice in these lines makes the speaker's fear feel very immediate. We can almost feel it and participate in the fear. The flashing dread feels sudden, surprising scary, almost like a flash of lightning. Throw in the word "horrify" and we start to think about all those things that scare us. Readers might even start flashing to ideas of their own deaths or things that horrify them. (See what we did there?)
This is a special way of being afraid No trick dispels. (21-22)
This fear of death isn't kids' stuff. It doesn't go away when the lights come on. The speaker tells us that he can't reason it away or fight it with religion or philosophy. This is an important moment in the poem because, in a sense, it predicts what the reader might be thinking: Hey, you should try [insert your preferred method of dealing with the finality of death]. The speaker isn't having it. None of those "tricks" work for him. He's still afraid.
[…] this is what we fear--no sight, no sound, No touch or taste or smell, nothing to think with, Nothing to love or link with. (27-29)
The speaker gets even more specific about his fears here. It's more than just death. It's the quality of being dead. The speaker can't stand the idea of the nothingness he believes death brings—no senses, no human contact or connection. It is the idea of nothingness that the speaker fears most. Consider how the speaker describes the darkness when he first wakes up: "Waking at four to soundless dark, I stare." Does this description remind you of anything? There's no sound. It's dark, so no sight. Feels a bit like the speaker's own description of death, doesn't it? No wonder this guy is freaked out.