Thanks again, gentlemen.
I did have great doubts that any significant pressure differential could exist, but the way the manuals are worded leads to confusion. As previously said, 25,000' is not what you would call comfortable.
After further reading, I discovered that the Flight Deck (aircon) Flow regulating valve is fully opened, rather than fully closed during a cargo fire, so perhaps there is a very slight positive differential which (helps) stop smoke entering the flight deck.
Cheers.
NSEU