There is clearly confusion on this subject. This is downloaded from the NPLG Website:
SECTION 2. CROSS-CREDITING LICENCES AND RATINGS TO NPPL (SEP)
2.1 Pilots with valid licences and ratings
NPPL (SLMG) or UK PPL (A) SLMG to NPPL (SEP)
The holder of a NPPL (SLMG) or UK PPL (A) SLMG who wishes to obtain a NPPL (SEP) shall:
a. Produce the NPPL (SLMG) or UK PPL (A) SLMG;
b. Produce log book evidence of having satisfactorily completed SEP differences training with a SEP FI in a simple single-engine piston aeroplane;
c Hold a valid NPPL Medical Declaration or JAR-FCL Class 1 or 2 medical certificate.
That is where your student gets his idea from regarding differences training.
Now read Schedule 8
Section 3 – National Private Pilot’s Licence (Aeroplanes)
National Private Pilot’s Licence (Aeroplanes)
Minimum age – 17 years
No maximum period of validity
Privileges and conditions:
(1) Subject to paragraphs (2), (3), (4), (5), (6) and (7) the holder of the licence shall be entitled to fly as pilot in command of any simple single engine aeroplane, microlight aeroplane or SLMG specified or otherwise falling within an aircraft rating included in the licence.
Differences training
(7) He shall not fly:
(a) as pilot in command of such a simple single engine aeroplane where:
(i) the aeroplane is fitted with a tricycle undercarriage;
(ii) the aeroplane is fitted with a tailwheel;
(iii) the engine is fitted with either a supercharger or turbo-charger;
(iv) the engine is fitted with a variable pitch propeller;
(v) the landing gear is retractable;
(vi) a cabin pressurisation system is fitted; or
(vii) the aeroplane has a maximum continuous cruising speed in excess of 140
knots indicated airspeed; unless appropriate differences training has been completed and recorded in his personal flying log book; or
(b) as pilot in command of such a microlight aeroplane where:
(i) the aeroplane has 3 axis controls and his previous training and experience
has only been in an aeroplane with flexwing controls; or
(ii) the aeroplane has flexwing controls and his previous training and
experience has only been in an aeroplane with 3 axis controls;
unless appropriate differences training has been completed and recorded in his
personal flying logbook.
There is no mention of SSEA under Differences Training!
It is necessary to have an SSEA Aircraft Rating added to the licence.
Article 28 says:
(2) Subject to paragraphs (3) and (4), the holder of a pilot's licence to which this article applies shall not be entitled to exercise the privileges of an aircraft rating contained in the licence on a flight unless the licence bears a valid certificate of test or a valid certificate of experience in respect of the rating, which certificate shall in either case be appropriate to the functions he is to perform on that flight in accordance with Section 1 of Part C of Schedule 8 and shall otherwise comply with that Section.
AIC 3/2004 says:
1.3 Sect on 3 of Part A of Schedule 8 of the Order states that the SSEA rating included in a NPPL may be validated by means of a continuous experience process or by means of a Skill Test. The Order does not stipulate a validity penod for the Skill Test so the CAA sets out below a clarification of its policy:
1.3.1 For the SSEA rating to be valid so that a pilot is entitled to exercise it's privileges on a flight, the licence holder must either:
(a) Have completed 6 hours flight time in the 12 month period preceding the flight including not less than 4 hours as pilot in command and have completed an instructional flight of at least 1 hour In the preceding 24 months; OR
(b) have undertaken and passed a Skill Test with an Authorised Examiner Whin the previous 12 months. The Examiner conducting the test will sign the Certificate of Revalidation Page of the NPPL and enter the date of the test but no explry date will be shown in view of the fact that validation may, in the future be maintained via the experience route
From the above I can only deduce that you must do a Skill Test and aply to have the SSEA added to the NPPL. Without the test the rating could never be valid!