No - you just don't hear the answers you WANT to hear. Post a picture that is clear, or don't bother seeking advice. Simples.
Service valves on boiler are universally crap. You have no idea what the various bits of the valve do and haven't clearly indicates which bit is leaking. Ergo, the advice given is the advice based on years of experience and the quality of the information given.
My last car's clutch was fine until my brother drove it. Did I blame him for it failing? No. I considered all the other variables other than the simple and obvious.
The "you touched it last" argument from customers is supremely annoying and seldom relevant; usually tried on by whinging little ingrates looking for a freebie.
It may be something he has broken, but so far I'm thinking, not so much.
Catch-22 I suppose. Easy cop-out to blame the equipment for your own mistakes though. Plumber might damage equipment while trying to rush the work and then he turns around and claims that it's standard wear and tear, absolving himself of all responsibility.