Nothing wrong with a master link per se, but original timing chains did not have them, nor do most replacements. Given the many problems with inferior replacement parts these days, it makes me really nervous buying internal engine parts from Joe's Screen Door and Triumph Parts Emporium.
If you use the chain with the ML, be sure to fit the open side of the clip away from the direction of travel.
Here's a shot from another Stag owner, who got burned by bad timing components. Unfortunately for him, the Stag is an interference engine, so the pistons met the valves with unpleasant results. A TR3 motor will just stop running.