edit: Thanks to /u/iring75 I found out how to adjust the temperature, unfortunately adjusting it fully either way does nothing. Guess there was more to it than it being mis-adjusted, will call plumber, thanks all.

I moved into a new place which has an issue with the hot water supply to the bath.

All hot water is fed from the same tank and they are fine except the hot tap for the bath which is barely lukewarm. The temperature of the hot tap in the bath does improve if the cold tap in the bath is run simultaneously.

The hot tap in the bath is fed via this thermostatic blending valve underneath.

The model number on it is Sirrus BVT15C T48. It can be found in this brochure as the 15mm blending valve.

Can any of you let me know how to adjust the temperature on it? The gold part in the picture looks like it can be turned by a wrench but I am worried that this would not be adjusting and instead would be removing it. There is also a small hole in the very centre but I have no idea what would go in there to adjust it.

Thank you guys so much for any advice you can offer.