The most expensive words in property are "it will need replacing". They are sometimes true. They are also what you say when you cannot find the fault, because replacement is the one recommendation that cannot be proved wrong: rip it all out and the problem does go away, along with the evidence.
A flat in Little Venice had a heating fault that had defeated everyone who looked at it and had followed the property through two sales. Every professional opinion was a full rewire and a new system. The fault was a concealed switch behind a partition wall. A shower valve in another flat was quoted as a stripped and retiled bathroom; it was a failed cartridge reached through an opening the size of a saucer.
In both cases the difference between the two recommendations was tens of thousands of pounds, and the thing that separated them was an afternoon of looking properly.
This is not a claim that replacement is always wrong. Things genuinely fail. It is a claim about sequence: that diagnosis is a separate piece of work from repair, that it deserves to be paid for on its own terms, and that a client is nearly always better off buying an investigation than buying a solution to a problem nobody has yet identified.
The uncomfortable part is that investigating properly is bad business for whoever does it. The plumber who finds a hidden switch bills a fraction of the plumber who installs a new system. That is precisely why so few people do it, and precisely why it is worth paying someone who will.
Diagnosis is a separate piece of work from repair.