Jasper.
All Group A ratings changed automatically to SEP on the first revalidation after 1 Jan 1999. The Group A rating no longer exists.
A Group A rating never entitled you to fly a SLMG, it has always been a separate rating, you had to pass a GFT on an SLMG in order to get a C of T and activate that part of a UK PPL.
The process is exactly the same now, you have to do a Skill Test (No navigation part)on a TMG to add that rating to a licence. You can add a TMG to either a JAA licence or a CAA licence. You cannot add a SLMG to a JAA licence, but you can still be issued with a National SLMG Licence.
One rating does not cover any other, they are entirely separate and stand alone. The only common point is that if you hold both SEP and TMG, then hours on either count towards revalidating both ratings.