Triton Opal Shower does not stop completely when I press the 'Stop' button?

Triton Opal Power Shower. When the stop button is pressed, the shower stops heating the water, and the flow of water is significantly reduced, but there is still a small flow of water coming through the shower head.

I would guess either a washer or seal has gone, and therefore the 'stop' mechanism is not functioning correctly. Can anyone suggest what this might be, and where in the shower the problem item may be found?

Any help would be greatly appreciated. Thank you!