After reviewing the 3 firemen testimonies (and a waiting captain's), I note 2 things:
First, even if the 3 firemen were in the same room, they don't give the same exact same location/timing for the events.
Example: event "blake smoke like a tyre exploding" at around S6 or S5 for two of the firemen, while the third didn't see the black smoke, but only the beginning of the fire, around S5, with first a small flame at the rear of the engine (different from the reheat), then the big flame everybody saw later.
Nevertheless, they were close enough (not like the foreman I remembered from a TV show) and their descriptions are precise enough to come to the conclusion that it seems the "black smoke" event took place at or shortly before the "first apparent trace of destroyed tyre" located between W7 and W6 on the appendix12.
Indeed "around S5" and "shortly after W7" are close enough - or were, in 2000, now the taxiway have different names.
Also noteworthy is the confusion in some TV shows when they said that witnesses had said the fire had begun a long distance (wasn't 1 km quoted?) before the strip. Yep. That is before where the strip was finally found. Not before where the strip probably was when Concorde rolled over it. I really have no difficulty to imagine the strip being swept along the trajectory of the aircraft, as its 200 tons rolled over it... but it seems that notion eludes some people anyway.
Second, many people said the BEA just "disregarded" the testimonies of the firemen. Not so, as seen above.
And quite not, when we read in extenso the appendix6. We understand then that the fact that the BEA was so keen to say the fire may have begun at the rear of the engines (by opposition to the second theory of the ignition in the U/C bay, which is the only one the AAIB considers valid) is somehow directly related to the testimonies of the firemen, which were trusted (as they should be) regarding the sequence of events (first this, then that: we human are good for that; not so when it came to precisely locate (in the space or time) this or that).