I have two of the long coats both with the single strap you mention.
On the opposite flap of the coat just below the position for storing the triangular neck flap is a button hole.
The strap will button to this, long or short depending on how big your gut is -or indeed how much extra clothing you have on.
It then keeps the middle part from flapping in the breeze.
I never had WD leggings so didn't consider that use of the strap.
The coat is a clever piece of kit and I travelled many everyday miles, snug and dry in mine much to the amusement of my pals who preferred to look "cool' -and wet!
David you are quite right there is a button hole lined up with the twin button strap which of course I can just do up, and that was obviously (now you mention it) its original purpose.
Nick the three buckle boots are much earlier than you may think, a friend of mine had a WW1 dated pair a few years ago, the toe was more like the earlier ammo boots without the external cap, they were used by the Royal Engineers but not necessarily for motor cyclists just muddy work.
My size 11 3 buckle boots fit in both of my sets of leggings, but I haven't tried it wearing them.