yes Pavel, of course you are right, the guitar was tuned half step DOWN not up
I guess Guy meant that Mark played it half step UP in term of position on the neck
I searched videos for my article and I found these ones :
Glasgow 8/10/2011 (2nd show of the tour) : Mark is joking about the song in Am. and he clearly hesitates for the first note
Nottingham 11/10/2011 : Mark goes back to the standard tuning, and he hesitates for the first note again !
oh and another think (thanks to Wolfgang) : of course tomorrow is BIA's
35th anniversary, not 25th as I wrote in my first post