Hi Robert,
I'd suspect a Music Diploma would be something you would consider once you had successfully completed your music grades up to grade 8 hence not something I know much about but with your experience it might be very rewarding.
This is a snipet from
Trinity College:
Guidance on Levels of Diplomas
Trinity Guildhall has 3 levels of diplomas:
Associate (e.g. ATCL, AMusTCL) - NQF Level 4
These diplomas are comparable with the standard reached at the end of a first year in a full-time undergraduate course at a conservatoire, university or other higher education institution.
Licentiate (e.g. LTCL, LMusTCL) - NQF Level 6
These diplomas are comparable with the standard reached at the end of the final year a full-time undergraduate course at a conservatoire, university or other higher education institution.
Fellowship (e.g. FTCL, FMusTCL) - NQF Level 7
Theses diplomas are comparable with the standard reached at the end of the postgraduate course at a conservatoire, university or other higher education institution.
I suspect Pete might be able to shed some more light on this as I think he used to teach on an undergraduate course.
All the best,
Chris