I haven’t been spending too much time in the XK forum lately and have been doing some catching up, so I’m chiming in late to this thread.
I, too, had the same issue and, like Roger, spent a bunch of time sitting on a stool in the empty engine bay trying to figure it out, though I had my wife at the grille end opening and closing the bonnet while I observed (I tried doing the observation bit solo, Roger, but raising the bonnet from its middle changes the dynamic.)
What I observed is, just as the bonnet is opened, the forward part of element “C” in Rob’s depiction drops about 1/8" relative to it’s closed position on link “D”. As the bonnet continues to be opened, element “C” swings back upward. I concluded that a stop silver soldered onto inside link “D” to keep “C” from dipping downward would solve the problem.
This is the stop - 1/8" x 1/8" x 1" - on the right hinge
And on the left hinge
The bonnet gap is an even 3/16" all around.
problem solved.