On the US tr7-and-tr8-forum they have been developing a 3D printed gear lever retainer over the past year or so. Knowing the problem I had replacing mine when it broke, this may be of interest to others. Its just been announced that the testing is finished and it will be available shortly:-
http://www.triumphexp.com/forum/tr7-and-tr8-forum.3/lt-77-gear-change-retainer.1762534.1890082/page-7#msg-1890082
Cheers
Howard