I don't think the black one below the silver valve is a temperature ragulator. It spins freely and makes noise when I spin it with a tap on, thats why I thought it had something to do with presure.
andrewNZ: FYI, the bulky component on the incoming pipe (centre of the third photo) is the Ajax valve. It regulates the incoming water pressure.
If you adjust this valve one of two things happens.
The silver one is the tempering valve. It has hot water from the cylinder on one side, cold water from the supply at the bottom, and feeds the taps on the other side. The black valve below the tempering valve is probably a regulator to adjust the temperature at the tap.
- Your hot water pressure gets worse
- You overflow the hot water onto your roof constantly meaning you'll possibly have no hot water and a huge power bill.
I suppose there is a chance that the tempering regulator has been adjusted and is causing this. The way my mind sees it, it could fit your symptoms exactly. BUT, I'm not a plumber.