Thanks for all the comments.
As it stands my Odyssey leg straps can go all the way down to my knees on a sit fly regardless of how much I tighten the legstraps (the padding overlaps by about 1" when tightened - leaving way too much excess strap to stow!). Anyway, very unhappy with the fit of my Odyssey and bereft of a response from Sunpath I will be adding the bungee cord :-(
One comment: If I remember correctly this method of controlling the legstraps originated as a retrofit for older rigs in order to make them freefly friendly. Surely it shouldn't be required on a rig specifically designed for freefly? I have an old Racer on which the leg straps don't budge at all during a sit fly - possibly due to it not being articulated like the Odyssey I just bought.
No, I can't continue to use the Racer as all rigs with any Velcro have been banned for freeflying in Ireland - debated ad nauseam.