The feltwell group

Ely Cathedral

Reproduced with the kind permission of Jonathan Rawle.

The topography of Norfolk being flat necessitated the use of triangulation stations on elevated positions.

Ely Cathedral was one of those used. I did not myself climb this tower but according to others one climbed inside one tower and then used a ladder to access the tower where the trig was situated. 

From the OS details are  Ely - TL5480 - On the roof of the tall West Tower, above the entrance.

Other elevated trigs were on Church towers and water towers. Water towers required the surveyor to climb up a ladder usually under the tank, along a walkway and finally up a ladder on the outside of the tank. The tops were curved surfaces which required great care by the surveyor.