UPDATE NOVEMBER 2020: The building above the wall has recently undergone renovation, which has installed new lighting and made nighttime ascents much more pleasant. however, they are concerned about people pissing in the doorway where they store their bikes. PUT SIMPLY, IF YOU KEEP PISSING HERE, YOU'RE JEOPARDISING ACCESS.
go down the lane and use the bushes if you can't hold it in. Their staff shouldn't need to wade through pissy leaves in order to go home.
If coming down Broomgrove Road (from Broomhill way), about 1/4 of the way down the road, and immediately after a large Hallam Uni building (Broomgrove Hall) there is a small driveway on the left with a yellow sign. The wall is on the building on the left at the end of this drive (50m down).
The hall above the wall is used by the university and the windows are often open so if you are using the wall during the day, keep the noise down.