I have a telephone style bath mixer tap with a lever selector, make unknown.
It's leaking past the valve when selected to tap position such that water
drips through the showerhead.

How does one access the valve washer?

Removing the lever shows a nylon body that moves up and down, but I can't
see how to get to it. I presume the shower rest unscrews but there is no nut
and I don't want to force it.

Jerry