Hourbuilder is not 100% correct. A TMG rating is a JAA rating and an SLMG rating is a National rating. You can get an SLMG rating by differences training, but any flying you then do in SLMGs won't count towards your JAR SEP class rating revalidation requirements. You can get a TMG rating by flying a class rating skills test with a JAA examiner (PPL examiner or class rating examiner) and then you can count the hours towards your JAR SEP class rating revalidation. Just to confuse things even more, there is a mechanism whereby you can convert an SLMG rating into a TMG rating if you have the right number of hours and apply to the CAA. LASORS has the details.
In case you're wondering, all TMGs are SLMGs but only SLMGs with non-retractable engines or propellers are TMGs.