Like I mentioned above, it was just what i read, i wasn't sure about it. It would've give the D-box = torsion box argument more weight, but I think even if it's not true, the evidence is enough.
I found the source in the meantime, but it's just an internet site, so not reliable. I'm posting it just for the interested, not as proof:
For someone, who's unaware (as I was up to about 5 years ago) how the treatment of the Spitfire changed, over the years, it's easy to be fooled into thinking that the l/e is a single piece.
Up to August, 1942, the paint manufacturers had been unable to come up with a genuinely smooth, while remaining matt, cellulose paint; Joe Smith did his own research, and found a synthetic paint which filled the requirements, and the Air ministry were sufficiently impressed to change the paint specification (later, Beaufighters Typhoons changed, as well, and there's a possibility it also applied to Lancasters, but that remains to be seen,) and, at the same time, called for improved finish in the area of the leading edges of the wings.
This involved filling the gap/crack on the extreme nose, plus all rivet "divots," with a stopper, rubbing it down, then priming, rubbing that down, then the final paint finish, all designed to give a completely smooth surface over the front 20% of the wing. This added 50 manhours to the job, but was felt to be worth it. According to an I.C.I. man this was extended to include the front portions of the tailplane and fin, at some later stage.
During the 1970s, I was involved in some voluntary work on a XVI (now somewhere in Southern California,) and, though I have no photos (never thought of it, and the paint stripper burnt like Hell,) I can attest to the seam, running the whole length of the wing.