Water hammer from unvented cylinder

[Taconite5]

Wondering if anyone might be able to spread some knowledge on what I might do next as this water hammer is doing my head in!

The hammering noise occurs in the region near to the multibloc on my unvented cylinder. It only seems to happen after using any hot tap for a bit (which doesn't have a hammer) then switching to cold. It then makes a banging noise/thumping noise when the cold tap is turned off and on. If I leave the taps for a bit and don't use them the noise will settle when using the cold tap. But return again when I use the hot tap first then switch to the cold taps again.

I've tried so far -

Getting the multibloc replaced (this is set at 3 bar)
Replacing the expansion vessel and checking it's charged (3 bar)
Fitting a pressure reducing valve to the mains (it was previously spiking to over 7 bar but typically was sitting around 4 bar) - now down at 3bar
Putting a water hammer arrestor before the multibloc (set at 3.5 bar)
Even tried replacing the cartridges in all the taps!

Any other ideas?

I've had a few G3 people look at it and they are stumped! Problem is I can't keep bringing them out just to have a look - so any ideas and where I can direct them would be helpful!

 

[dilalio]

What make and model uv?
What make and model control group?
Is the hot and cold balanced and both taken off of the control group?
Is the EV taken from the correct port of the control group?
Have you tried depleting the cylinder of hot water and seeing if problem occurs with a cold tank?
Some photos would be useful.