This particular part may be kicked around on the internet more than any other, because it is one of the most visible pieces on the car. There have been many problems with aftermarket replacement grilles for many years. The phrases "poor quality" and "bad fit" are very common in such discussions. Fairly recently there has been some glimmer of a better light. In particular, Moss Motors has recently gone to great efforts to reproduce a very original style grille with chrome over brass shell, stainless steel vent slats, correct number of brackets in all the right places, and generally correct as original shape. So far this is for the 1500/1600 style only, no 1600-MK-II type yet.

The following pages contain more specific comments on grilles form various sources.
Jaguar by George
Moss Motors
Scarborough Faire
Tasker Metal Products