The plate in the slam panel will have very minimal adjustment with the screws loose, and you will also be able to adjust the positioning on the striker where it bolts to the bonnet, and you can also adjust the tension of the spring on the striker.

Have a look at the catch in the slam panel you may find its worn and thats why, but you shouldn't need to fit any other means a standard catch when working properly is very good.

Mine hasn't flown up on the rare times ive taken it round to its max of 130mph (touch wood now ive jinxed it )