If you have a JAR-FCL SEP Class Rating and current UK national SLMG privileges (check in your licence), and you have flown a total of 75 hours total time as pilot of aeroplanes (including motor gliders which meet the JAR definition of TMG) , you can apply for the TMG rating without having to pass the TMG Licensing Skill Test.
Without a TMG Class Rating, you can't count hours flown under national SLMG privileges (even in a JAR-definition TMG) towards the re-validation of your SEP Class Rating.
So I guess that the answer is to obtain the TMG Rating (but there'll no doubt be a £67-ish fee to pay) if you have 75 hours TT, then you can count any TMG time towards re-validating both your SEP and TMG ratings.
See LASORS Section F5.3 and Section F1.4