Grohe thermostatic shower: leak from 'stat adjustment knob

I'm trying to help an elderly neighbour with their thermostatic shower, an inset thermostatic Grohe model Grohetherm 3000. The 'thermo element' replacement thermostatic insert is expensive, so I want to make sure I understand what is going on before suggesting replacements.
Basically, the thermostatic control is leaking; when the chrome tap cover is removed, there is a black plastic splined 'adjusting nut' (Grohe's description) which screws onto the thermostatic cartridge. There is a small 2mm diameter hole in this part, through which the water is leaking. Is this some sort of air release valve which is letting by? I can't really understand why there is a hole here!
Can anyone explain the function of this hole, and what I should be looking to repair/replace to cure the leak? The shower is working OK otherwise, and is about ten years old, in a pretty hard water area (London).
Thanks in advance.

I have just worked out how to upload images onto the site, so I have attached a couple of images of the leak streaming from the black thermostat adjustment knob:
and

The observant will notice there is also a small leak from the main tap above aswell..
Anyone with any experience of Grohe showers?
Cheers
Phil