The gap at the front is normally larger than the gap at the rear, designed that way to allow the front end to dip below the valance without touching.
Have you tried measuring the bonnet hole across the corners, it's possible past accident damage has made the hole trapezoid rather than square (OK rectangular, or not even that, whatever, you know what I mean!)
As Richard says, panel fit is a bugbear all Triumph owners deal with. I had the body shop jig straighten and test fit all the panels on my Sprint. The laser guided jig fixed the gaps to .5 mm and it all looked perfect. Then I subbed in a better bonnet afterwards, the one that I let the bulge into and it fits where it touches, high one side, low the other, twisted across the corners and very difficult to shut. Too late to go back now the paint is on, shimming the hinges may help when I get time, it's on the to-do list!
Yes i have measured from corner to corner, the car is square, dont know if the car has had damage before, going to have another look tomorrow to see if i can get a better fit, ive got another bonnet this one fits better than the other