Vaillant system banging and loss of water pressure.

[nutmix]

About 4 years back we had this system installed Vaillant auroSTEP pro/2 VIH S 300/2 T.

They did not install an expansion tank nor any connection between hot and cold which is shown on this diagram (sorry, I could not find a way to embed images into the post)

https://1drv.ms/u/s!ArKPx05MpLzblZkmQQzuOQp7Muv52Q?e=8sjae3

They installed the panels, tank and direct connection into the hot water pipes via tap and what looks like swing type one way (check) valves, with a cap (for inspection?).  1/2".

From the first day, opening and closing any cold tap results in very loud banging which lasts around 20 seconds. So loud it sounds like someone is starting a harley in your bedroom.

As the water was too hot, I installed a thermal mixer before the hot water comes into the house, which miraculously cured the banging.

The main problem is, however hot water pressure.  It starts of adequate (but half that of the cold taps), but goes down to a trickle which is not possible to shower with.  Because of lack of hot water pressure, I had to blank out the cold input on the thermostatic water mixer in installed, as it was coming through cold. THis fixes the hot water being cold, but now we have the banging back, and hot water is a trickle.

on the tank on the roof I cant see any type of filter or gauze to clean.  There is a pressure relief valve on the top of the tank, but opening this by turning it a bit doesnt do anything (I expected air or water to come out, but nothing does). 

The cowboys also installed it with a non working heater element, and no sacrificial anode, which I found when I went up to replace the heater element (and I installed one)

I guess the problems could include:

1) air lock.  Any idea how I purge this?
2) blockage.  Any idea how I might locate/fix this?
3) missing system components.  The diagram shows some sort of 3 way valve between hot and code. Does anyone know what this is?

ANy idea why there would be such banging when I open a cold tap? I guess this could be the swing type return valve on the roof (either hot or cold), but i have no idea why this would happen or how to fix.

My supply has 6 bar of pressure, and I have pressure reduction valve set to 3.8 bar as the water comes into the house.  This pressure never changes.

I called up vaillant. They charged me €70 to tell me that I would have to pay €270 for a man to go onto the roof just to look, then pay an unknown amount for someone else to come and actually fix it.  I declined.