"I took the camera to a [b]repair shop[/b] (Aperture UK in London) and they assured me the [b]camera[/b] was[b] in perfect working order[/b]. ...
N.B. [b]the ground glass is not upside down[/b] in the viewfinder. The [b]pressure plate is set correctly[/b]. The readings on the [b]focus knob appear to be accurate[/b]."
Now this is making it a tough one.
If I hat this kind of trouble ... I'd recheck my eyes
check the camera for obvious damage (set to a short distance and see if the lens/shutter cover to camera body distance/gap is the same top left right bottom),
try to turn ("screw in") the taking lens's front element (where the writing is).
Later on, I'd take a ground glass and put it where the film usually goes,
set f 3,5, open shutter at "B", lock it open,
put an 8x or 10x magnifier on the ground glass in the film plane and recheck in the viewfinder.
Check at "infinity" (something 200m away or more),
OK or "off"?
"off"? Go back to Aperture UK in London and tell them to do their job properly (make sure, lens plane is parallal to film plane, make sure, taking lens will reach "infinity" and focus knob is adjusted accordingly, make sure, finder lens is properly collimated (and focusing screen is put in the right way and all the way etc.)).
"OK"? A lot of head scratching ...