The pipe is a 21-inch pipe according to this UKPress article. 53 centimeters is about 21 inches.

The article seems to be inaccurate in at least one respect, and one comment calls the author on it: It's not a 5-foot diameter pipe. Various sources say it's either 12-inch or 21-inch, but not five feet. One source says the largest riser pipe made is 21-inches in diameter.