Final Update
Even for more modern JAA licenses, a SLMG rating can be added. There are therefore 2 routes to putting a TMG rating on your PPL:
1. TMG rating direct with a TMG examiner (which are rare).
2. SLMG rating then convert to TMG rating based on experience. A SLMG examiner can perform the necessary class rating skill test. Note you will not be able to log any hourse until you have a TMG rating on your license.
It would seem the NPPL route is no longer required, unless you have a desire to hold a NPPL for other reasons.
The CAA policy section has confirmed the following:
To apply for the grant of a TMG rating to your JAA PPL(A) on the basis of having flown at least 75 hours as pilot of aeroplanes, as detailed in section F1.2 of LASORS, without passing a TMG skills test. You are required to hold a UK issued PPL(A) or above which contains both an SEP land and an SLMG rating with a valid certificate of test for SLMG's. A UK issues licence can be either a JAA PPL(A) or a previously issued National UK PPL(A), these were issued pre JAR.
Your options are either to add a the TMG rating onto your licence as detailed in LASORS section F1.2
Provide evidence of having completed a course of training in a TMG in accordance with JAR_FCL for the purpose of the rating.
Pass a class rating skills test in the appropriate class of aeroplane conducted by a JAR authorised Class rating Examiner or Flight Examiner.
Pass a theoretical Knowledge exam (oral as part of the LST).
Or alternatively you can meet the requirements to have an SLMG added to your JAA PPL(A) by meeting the requirements for the addition of this rating as detailed in LASORS Section F5.2. The application and test for the SLMG rating can be signed by an SLMG examiner. Once this has been completed you can then subsequently apply for the grant of the TMG rating based on the 75 hour basis.