As I said, you will be doing conversion training for your SSEA Class Rating, not 'differences' training.
If you hold a NPPL with SLMG and SSEA Class Ratings, but have only flown, for example, 3 hours with a JAR-FCL-appointed FI and a couple of hours supervised PIC time in the circuit during SSEA conversion training , then to upgrade to a JAR-FCL PPL(A) you will need to achieve a further 17 hours of SEP flight training with a JAR-FCL appointed FI, and further 8 hours supervised solo time (including 5 hrs cross-country which must include the 150nm qualifying cross-country) on SEP aircraft. That would add up to 3+17+2+8=30 hours on SEP aircraft, so you will then need a further 5 hours on SEP aircraft (which can be dual, solo or combination of both) to give a minimum total of 35 hours SEP time under the auspices of JAR-FCL training. You need a minimum total flight time of 45 hours before applying, so 10 of those further hours could have been flown on SLMGs. You then take the JAR-FCL PPL(A) Skill Test (not the 'PPL GFT' - there's no such thing).
It might seem excessive, but the JAR-FCL 20 hrs dual and 10 hrs PIC are mandatory requirements which the CAA has no powers to waive. The Authority agreed these 'conversion' requirements with the NPPL Policy and Steering Committee and they are quite reasonable.
We have no idea what lunatic Colgonic irritations will be introduced by the €urocrats at EASA as they haven't even considered licence conversions yet.