Differences training is required for a TMG class rating in accordance with FCL.710:
FCL.710 Class and type ratings – variants
(a) In order to extend his/her privileges to another variant of aeroplane within one class or type rating, the pilot shall undertake differences or familiarisation training. In the case of variants within a type rating, the differences or familiarisation training shall include the relevant elements defined in the operational suitability data established in accordance with Part-21.
(b) If the variant has not been flown within a period of 2 years following the differences training, further differences training or a proficiency check in that variant shall be required to maintain the privileges, except for types or variants within the single-engine piston and TMG class ratings.
(c) The differences training shall be entered in the pilot’s logbook or equivalent record and signed by the instructor as appropriate.
ANO Article 67 cross-refers to FCL.710 which means that a TMG class rating in a UK national licence (such as the old UK PPL) has the same requirement. That said, when you look at GM1 FCL.700 it states the text Whopity has provided above which does not specify any differences as such for TMGs.
For a SLMG class rating in a UK national licence such as the old UK PPL or NPPL, as provided by Whopity above, ANO Schedule 7 Part B Section 2 applies which does not require any differences training. That's not to say it isn't a good idea!
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