My last post was wrong. The bottom sign is 20 steps above the platform.

There are three groups of steps: 18, 48 and 20. As Helen says, the lift
bypasses the top 66.

I still do not understand why the warning signs are the way they are.
Perhaps they used two different sign contractors. Or maybe it is a joke
in tribute to Willie Rushton (there is a plaque to him at the top of
the stairs too).