[Leica] 135mm practice, using flash on an M6

[geordiepete211 at yahoo.co.uk (Peter Cheyne)]

A friend is getting married at a Shinto shrine next year, and he'd  
like me to take some photos to complement the
photographer he'll hire.  I'll bring a 35 nokton, 50 'cron and a 135  
3.5 (Canon LTM).  I haven't used the 135 much,
but I'll bring it to use just a few time to get some close-ups.  So  
before the April wedding, I intend to practice with a few rolls
using the 135.  The practice is mainly to get a feel for this lens as  
it a bit strange to use.  The barrel rotates, and the aperture dial
is at the end of the barrel, meaning that when I change the aperture,  
it's easy to unintentionally adjust the focus.  I'm not very worried
about focussing with the 135 frame lines, as the, admittedly few,  
photos I've taken with it so far are nicely in focus.  ?? I might  
change my mind
about that after I've exposed and developed a couple of rolls.

Usually I'd use an SLR if I wanted to use a long lens.  But I'd prefer  
the comfort and lightness of a rangefinder as guest to my friend's  
wedding.
I won't bring a flash to the wedding.  But in order to get a feel for  
the lens, I'll practice taking photos of my daughter indoors,
needing the flash for that.  I tried my auto-thyristor Canon 580EXII  
on the M6 (not a well-balanced pairing) a few times, but each time the  
flash didn't fire.
Pressing the pilot button fired the flash, so the flash is fine.   
Flash was set manually, on auto-thyristor for the desired aperture (f/ 
3.5).
Does anyone know if a flash cable is needed to use that flash on the  
M6?  Or have I just set something wrongly?

Peter Cheyne