Any idea if this is some kind of tap

[regfixit]

Context is problems with shower. Starts warm, then goes cold and repeats. Have also noticed hot top also does this. I think the HW flow rate is too low. I thought at first maybe just crappy mains pressure, but noticed that hot water flow rate much lower than cold and so maybe boiler heat exchanger is clogged or there is some kind of flow restrictor fitter. It is my girlfriend's house. Boiler is Vaillant, a few years old but not ancient. I suspect this is some kind of tap to turn the flow rate down, but I've not seen anything like it and looks old school.
Before I meddle with it thought I'd ask here if that is what it looks like. Also, which bit would I turn - the screw head, the smaller nut, etc ?

 

[picasso]

The bit you have circled is a gas test point so best leave that alone.... what model vaillant is that ?

 

[regfixit]

Ah, glad I asked thanks ! Won't try to unscrew that then
I'm not sure the model number, not back there until Sunday so will check then.

 

[regfixit]

It's an eco max 824/2 E. I found the cold inlet pipe and it did indeed have a valve restricting the flow. The flow rate was about 3 l/min. I opened the valve a bit to increase this. However, then it seemed to produce no hot at all despite then turning it down again. Moving the HW temperature control full up seemed to get it to come to life, but it all seems a bit intermittent. There was a flashing C in the panel and it the temp on the boiler went down after hot tap turned on, then gradually climbed back up to 65. After turning taps off it seemed boiler kept going and went up as high as 85 before shutting off. I didn't notice the tap symbol in the display.
I suspect there is some kind of fault with the flow sensor.
Getting a Gas safe chap to it on Monday and hoping to limp along until then.