Speedy Jim wrote:
The black rubber ring is the seal between pipe and drain fitting.
It can be driven further down into the annular space.
You can use a tool like a thinwall cylinder or argh a blunt
screwdriver. But if it doesn't move fairly easily, there
may be too much friction built up.

Thanks. Do you know how much space there is below the (current position
of the) rubber ring to allow it to be drive down?