Plughole - Non Return

[Granty]

It would seem the bathroom in my place was done on the cheap. 

When you let water out of the sink/bath, the other plughole gurgles and water sometimes travels up
 
Should a non return valve (or similar) not have been fitted?  Is this an easy job?

[Mantis]

The bubbles are because the water in the pipe creates a partial vacuum behind.

It is probably the gradient of the pipe that is the problem. I think it needs to be steeper.