Shower attached to mixer tap won't shut off

[grovern]

Hi,

The shower attached to the mixer tap does not close properly and water keeps running slowly but steadily. I opened the cap to see if there is something I can replace but not sure I am able to figure out. Please see below

1. Shower and tap with cap removed:
View media item 44653
2. Shower knob close up:
View media item 44654
Apologies for images wrong way round. Somehow website is changing the orientation.

The mixer shower tap is from Tre Mercati.

Thanks for your time.

Neeraj

 

[grovern]

Please any help? I am still confused how to replace the valve or cartridge.