Water Hammer

Reading through this, I’d be looking beyond the usual water-hammer fixes. The fact that you’ve replaced so many components, reduced the pressure from 4 bar to 2 bar, and still have loud banging — particularly when turning taps on — suggests there may be an underlying flow restriction, pressure transient or pipe movement that hasn’t been identified.
Here’s what I’d investigate next.
The five most likely causes
1. A restriction or faulty valve on the incoming mains
A partially obstructed stopcock, pressure-reducing valve, check valve or another fitting can create turbulent flow or abnormal pressure behaviour. A pressure reading of 2 bar at rest doesn't tell you what happens when a tap opens.
Measure dynamic pressure while a tap is running.
Check whether the pressure-reducing valve is installed in the correct direction and is suitable for the flow rate.
If practical, have a plumber test pressure before and after the PRV and check for a significant pressure drop.
2. A pipe moving when flow starts
Although you've clipped the pipes every 50 cm, a long pipe run can still move if the clips are loose, the pipe is expanding against a fitting, or a pipe passes through a tight hole in a joist or wall.
The sudden noise when a tap opens could be caused by a pipe jumping against timber or masonry as the flow changes. Have someone operate the taps individually while you locate the exact point of impact.
3. A problem associated with the pipe you removed
This is particularly interesting because the noise started after you removed one of the two connected 22 mm hot-water pipes.
I'd want to establish exactly what those pipes did. They may have formed a redundant loop, a circulation arrangement, or another connection that was part of the original hot-water layout. If removing one left an incorrectly capped branch, an unusual connection or an altered flow path, it could be relevant.
The timing doesn't prove that removing the pipe caused the hammer, but it makes tracing that alteration a priority.
4. A faulty or unsuitable check valve
Check valves can cause pressure fluctuations if they close abruptly or interact badly with other valves. Since you've already fitted one, I wouldn't add another. I'd check whether it is actually required, whether it is the correct type, and whether it could be contributing to the noise. Any change should preserve the necessary backflow protection.
5. A pressure transient elsewhere in the house
The washing machine, dishwasher and toilet fill valve can all change water flow quickly. If the banging happens when one of these appliances starts filling, rather than when a tap is opened, that distinction could identify the source.
Are your two theories correct?
Horizontal versus vertical pipes: Water doesn't behave in a way that inherently causes water hammer simply because a pipe runs horizontally or vertically. However, long vertical runs, changes in direction, restrictions and poorly supported pipework can affect pressure transients and make noise worse.
An airlock at the boiler: Unlikely to be the primary cause of this particular symptom. A combi boiler's sealed heating circuit is separate from its mains-fed domestic water circuit. Air in the domestic pipework can cause spluttering, but it doesn't usually explain persistent, loud banging every time a tap is opened.
What I'd do before spending more money
I'd stop replacing parts and carry out a controlled diagnosis:
Identify whether the noise occurs on hot water, cold water or both.
Test each outlet separately, beginning with the kitchen cold tap, then the hot tap, bathroom taps, bath and toilet.
Determine whether the noise starts immediately when the tap opens, while the water is running, or when it closes.
Measure static and dynamic pressure, ideally also checking the pressure behaviour at the incoming main.
Trace the old 22 mm pipe arrangement and inspect the route for restrictions, unusual branches and concealed contact points.
If the noise is extremely violent, or you see pipes jumping, leaking or fittings moving, stop using the affected outlet until it has been inspected.

Hi. Water hammer is normally caused by a washer/faulty tap body.
When turning on a tap in the property or flushing the toilet can cause water hammer to occur. This is because all the cold water supplies are linked and so are the hot water supplies.
I would turn each tap on and off individually in order to locate which one needs a new washer or body.
When turning on a tap if one of the washers is faulty you will get a loud banging noise which is caused by the washer spinning uncontrollably.
I would focus on when the noise occurs to replace that taps washer/body.
Can be a bit tricky to identify but that is normally the cause of water hammer. Maybe even the inlet valve washer to the toilet could also be the cause. Hopefully this has narrowed it down for you and you should be able to solve the problem when focusing on the taps washers or bodies.

Before spending heavy amounts of money, test these targeted diagnostic paths based on your system layout:1. Investigate the Remnants of the 22mm PipeGo back to the exact location where you removed that 22mm pipe.Did you cap it off right at the T-junction, or does a piece of pipe still extend into the wall/floor before the cap?If there is an unvented stub (even a few inches long), it is acting as a waterlogged air pocket or a dead leg. To fix this, any unused pipework must be cut back and capped completely flush with the active flowing main line.2. Re-evaluate the Water Hammer Arrestor PlacementArrestors only work if they are placed immediately adjacent to the problem area. If you installed multiple arrestors on the main rising line or in the loft, they won't stop a shockwave originating in the bathroom.Try installing a mini-arrestor directly underneath the bathroom sink or bath tap supply lines (the closest point to where you altered the pipework).3. Test for Shared Mains Interference (The Terrace Factor)Because you live in a mid-terraced house, your mains supply might be shared with your neighbors under the floorboard or out in the street.When you open a tap, you drastically drop the local pressure. If a neighbor has a faulty, loose jumper valve in their old stopcock, or an old ballvalve in their loft, your pressure drop can cause their valve to vibrate violently, telegraphing the sound straight through the party walls into your home.Test: Ask your immediate neighbors to shut their main internal stopcocks for 10 minutes while you test your taps. If the noise vanishes, the root cause is actually on their side of the boundary.4. Balance the System via Flow RestrictionsIf the bang is caused by water moving too quickly when a tap opens, reducing the volume of flow can stop it. You have already lowered the static pressure to 2 bar, but the dynamic flow speed might still be too fast.Try turning down the full-flow isolator valves under your sinks or bath to about 50% capacity. Open the tap. If the bang disappears or softens, the system is suffering from high velocity, and installing inline flow restrictors will solve it permanently.

I think there’s about an 80–90% chance the removed 22 mm pipe is directly related to the problem. The timing is too coincidental, and you’ve already ruled out almost every other common cause. If you can draw the original arrangement of those two pipes (even a rough sketch), I can probably explain exactly what their purpose was and whether replacing the second pipe is the proper fix.

Strange one...have you tried fitting an automatic air vent at all?