Banging noise in central heating system

what do you think the problem might be?

Hard to say definitively. Just strange that you've had what looks like a tidy installation but after a few posts your plumbers are now cowboys, they haven't a clue what they are doing and you have lost confidence in them!!
 
I can also see on your photos that the vertical pipework has been market H, C, F, R (hot cold flow return) so it seems, on the face of it, they made the effort to identify things correctly. Things do sometimes go wrong though no matter how hard you try.
 
I'm not judging my plumber based on a few posts on this forum

My confidence in my plumber isn't high for a numbers of reasons

1) It took me a year of chasing to get him to complete the commissioning document. He didn't register the boiler with Vaillant so my 10yr warranty was worthless. And installing without commissioning - is that even legal?

2) I had a poor installation of two wall radiators which caused a dripping shower sound that made the room unusable for months. He came back to fix it but didn't - I sorted it out myself.

3) I had a random banging sound - really loud- in the upstairs bathroom near the boiler and he said it was water hammer. He adjusted the mains pressure but it didn't fix the problem. I still have that problem but not discussed on this forum.

4) And I have these noisy radiators/pipes I didn't have with the old boiler that wake me up in the morning and noise continues regularly throughout the day. Installers solution was to turn down the power because 'it's really powerful' using his words to 24KW which makes me wonder why he installed a 37KW in the first place. He also increased the pump speed which which made the boiler so noisy I could hear it the other side of the house. And the adjustments made no difference so I've turned to this forum to get some ideas about what to do next.
 
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