Test has to be undertaken by a JAR EXAMINER not an FI.
The situation has changed since the days that you mention.
There are two distinct Instructor Ratings available through the BGA/CAA.
The first is the BGA MGIR - this permits the holder of a Full BGA Glider instructor rating with a PPL SLMG to instruct in Motor Gliders for the sole purpose of teaching gliding exercises. The rating is gained by examination by a suitably qualified BGA Regional or Senior Regional examiner.
The CAA MGIR is obtained by attending a one week course run by the BGA and permits the holder to instruct for the purpose of the student gaining an NPPL. This rating does not permit gliding instruction.
In addition one can always go the full CAA route and obtain an FI rating which would permit training for the TMG and SLMG ratings assuming that you hold these class ratings on your JAR licence.
There are various exemptions that permit training for the NPPL SLMG to be undertaken from specified BGA (non licensed) and also permit a SLMG instructor to charge for the training without the requirement to hold a CPL. - Most of this can be gleaned from LASORS.