Smelly Shower!!

Hi everyone, be gentle with me I'm a Newby!! The problem we have is we are getting a "Smell" coming back up through our Shower waste,the trap itself doesn't seem to hold a lot of water ( about 10mm) so therefore the outlet is above this.It's completely clean as is the pipework,got a good fall and runs away quickly Its a 90mm trap. Is there a product that will guarantee absolutely no smells? I don't mind changing it if it works or even some kind of "In line" trap? Thanks any way guys all ideas welcome !

Can you post a picture of the trap and the pipe leading away from it?