I think all the trades are now contaminated by dodgy unscrupulous individuals who just get everyone else a bad name in their quest for a fast buck. They want to get the job done, take the money and onto the next, I always found it better to explain things to people and also that sometimes a diagnosis might be wrong because it could be hidden by another fault. Just hoodwinking people with technical jargon is not in my opinion the way to represent your trade and get returning customers.
If I turned up at Cozzers house I would proceed in a logical manner.
1) Listen to his story and experiences.
2) From this I would be checking the mains pressure and listening to the noises to determine what it actually is, maybe water hammer.
3) I would not proceed onto the boiler issues until I had the mains pressure / stop **** resolved as this is a potential issue for all else.
4) When the mains pressure / noises have been resolved I have something to move forward with and have now discounted high pressure / low flow from causing any other issues.
5) Next is to turn off the isolators on the filter, (why is the filter housing spanner left there ? )
6) Remove the filling loop as it looks like this gives me access to the return pipe to the boiler and attach pressure tester.
7) Find a suitable location to isolate the boiler feed to the rads.
8) Pressure this part of the system and leave for at least four hours or overnight.
9) If pressure has fallen then your issue is within this section, if not move down the pipework and get to the last radiator you have access to.
10) At this radiator you need to isolate the pipework from going any further so all you have is that last rad with a feed and return in the circuit and nothing else. You can now remove isolation in 7) and pressure test again as before.
11) If pressure has not held then again you have found a problem otherwise this part of your system is sound.
12) Fit pressure tester to other part of the system using feed and blanking the return, now pressurise to 3 bar and wait.
14) If the pressure holds then you need to do a big rethink and start looking at your system in a lot more detail, where else do pipes run and use a listening rod to attempt to trace the leak under floors.
15) All else has failed and you have a leak in the last part of your system, if I was convinced I had fully tested and done everything possible and that the leak can only be under the concrete floor then first thing to look at is where any pipes exit the floor, keep the system pressurised and then start breaking out the concrete, get an hydraulic breaker as it will make the job easier. The better option if possible would be to fit new pipework above the concrete in false skirting boards and such to save time and cost in digging the floor out which could lead to other issues such as damp problems.