Well this makes no sense:
Me: "Hi, your letter said I needed to arrange a visit with you to check the gas safety of my flat."
Them: "That's right, what's your details?"
Me: "There's no gas supply in the building, but apparently you have to come see that in person yourselves."
Them: "Yes, that's true. .... OK, we'll send you a letter with the appointment details."
Me: "Uh, I can't just do any time, it needs to be arranged."
Them: "That's OK - if you can't make the appointment on the letter, then you can ring us up after you receive it and tell us."