I have to demonstrate to a FIC instructor with a TMG rating, that ability
No you do not!
FCL.905.CRI CRI — Privileges and conditions
(b) The privileges of a CRI are restricted to the class or type of aeroplane in which the instructor assessment of competence was taken. The privileges of the CRI shall be extended to further classes or types when the CRI has completed, within the last 12 months:
(1) 15 hours flight time as PIC on aeroplanes of the applicable class or type of aeroplane;
(2) one training flight from the right hand seat under the supervision of another CRI or FI qualified for that class or type occupying the other pilot’s seat.