Part is missing from my bath shower mixer tap?

Your question leaves out important information.

The part you seek is very probably called a selector or something similar - to direct the water to either the tap outlet or to the flexible hose and the shower head - the control handle of a two way valve.

First the manufacturer - then the model - then the part itself.

You might consider using Google and selecting images and enter the phrase Shower Mixer Tap

When you see an image similar to your mixer follow the link for more information.

Then contact the manufacturer for parts availability.

Alternatively - search the house to find out where you put it.

Good Luck...
November 2015