If you qualified for a PPL Aeroplane, the licence was issued with two non expiring ratings on the licence:
Landplanes in Group A and
Self Launching Motor Gliders
They were non expiring however; to use them you needed either a Certificate of Test or a Certificate of Experience (Art 157).
There is no such thing as differences training between different Classes. The classes are defined in the ANO Schedule 4 Part 1 (formerly Schedule 2)
If you had never flown a SLMG, all you needed was a GST and the Examiner entered the Certificate of Test in your log book, no CAA action required.
If you still hold a valid UK PPL then that process is still good today. ANO Schedule 8 Part 3
If you converted to an EASA licence and obtained a parallel National Licence, when the SLMG was not current, the CAA incorrectly placed the SLMG on the back. To transfer it onto the front, the CAA make a charge so, if you still have your old brown piece of card that does not say its expired, been calcelled or revoiked, then you could still use it.
Group A also included Group D (Microlights)
Examiners were allowed to change a Group A landplane rating to a SEP Class but oddly, not permitted to change a SLMG to the JAA equivalent TMG; some however did !