I have a radiator which is now on most of the day and I have noticed
that the wallpaper directly behind it has completely split in a very
bad way. This is presumably as a direct result of the heat.

Could be the result of drying clothes on the radiator. In the past, I have
painstakingly removed a rad and papered behind, only later to have the paper
peel in the way you describe. We used to be in the habit of drying clothes
over the radiator.

Moisture plus heat = wallpaper stripper