Easy way first ...
If your PPL has the SLMG attached as my UK PPL A had (I believe the early JAR PPL's still did) then all you need to do is a one hour flight with an SLMG instructor to validate it. If you have more than 75 hours P1 on either SEP or SLMG you can then send off to the CAA, pay a fee and have the TMG rating attached. Hours in a SLMG/TMG can from then on be used to keep your TMG and SEP ratings current. You cannot use SLMG/TMG hours to keep your SEP rating current if you do not have the TMG rating.
If your PPL does not have an attached SLMG then there are two routes as far as I can see. If you are already current on SLMG's (ATC instructor, NPPL SLMG) you can get an SLMG attached to your JAR PPL and add a TMG rating afterwards if you have the required 75 hours, or you can take a TMG skill test with a TMG examiner. The difficulty with that is there are only a few TMG examiners around, whereas there are loads of BGA/NPPL SLMG instructors who can sign you off for a SLMG rating. I'm not sure but you may be able to just do differences traing with an SLMG instructor to get an SLMG attached to your licence, but of course the hours wouldn't count until this is uprated to a TMG (which if you have 75 hours on either, you can do).
SS