If you wish to be able to count motor glider time towards revalidation requirements for the SEP Class Rating, you need a JAR-FCL TMG Class Rating - which requires you to pass a LST on TMGs with an appropriately qualified Examiner.
You will then be able to count all flight time on either SEP or TMG Class aeroplanes towrds the revalidation of both Class Ratings.
If this is of no concern and you don't wish to take a test, you can fly the aircraft as a SLMG, subject to completion of appropriate differences training, by exercise the privileges of the SEP Class Rating included in your licence on SLMGs. This diffrences training must be with a Flight Instructor holding a TMG rating or Motor Glider Instructor rating. However, any experience gained in SLMGs cannot then be counted towards SEP revalidation requirements.
So,
If you want to be able to count hours on either Class towards the revalidation of both, take the LST and obtain a TMG Class Rating.
If you don't wish to count the hours in this way, just do some differences training. No test, no additional Class Rating and no CAA fee...... Although if you really want the words 'SLMG Class Rating' in your licence, you can elect to apply to the CAA for a SLMG Class Rating to be included in your licence. Why would you do this? Perhaps if you have no further intention of flying a SEP Class aeroplane - such as an airline pilot who just wants to potter about in a SLMG for fun.