Having got one of the first SEPL ratings to issued in early 1999 (should I be proud of that?), I subsequently added a TMG later that year. It was confusing then and there still seem to be some grey areas. Don't want to put myself up as the complete expert here, but I and a friend of mine did go around the houses on this quite a bit (including a period of almost flying illegally without a valid rating).
It makes a difference whether you do in fact have a traditional "Group A" rating on your licence (ie. you completed your PPL before Jan 1 1999), or whether you have a "Single engine, piston, land" (SEPL) rating, either on a UK PPL or on a JAR licence. It's the type of rating you have that's important. An SEPL does not automatically replace an existing Group A.
If you have a SEPL, then you will need to do a skill test (which replaces the old NFT & GFT) following a "period of training" doing such things as described earlier. In practice, it seems that just the GFT bit is tested. You'll then have to apply to the CAA for the addition of the TMG rating using the appropriate form, enclosing the inevitable fee! (was £63). The CAA will then add the rating to the ratings page in your licence.
If you have a "Group A" rating, then in theory this already includes the SLMG and all you need to do is get the log book signed or whatever - no need to involve the CAA.
I don't think you have choice - SLMG is not recognised under JAR so you can't add it to an SEPL, and don't THINK you can add a TMG to a Group A.
Not sure how you do this, but if you could convert your Group A to a SEPL, then the benefit is that the SEPL and TMG ratings are the only two ratings where the CAA re-validation requirements are mutually inclusive - ie. you can maintain one with the other.
That's the theory, and its what the CAA blurb seems to suggest, but in practice, it has proved difficult to clarify the rules exactly, and find someone happy to sign one off on the strength of the other.
There is still grey areas - when I did the TMG test, there weren't any qualified TMG examiners, and I suspect that still may be the case. Issue was that most of the SLMG guys were doing it under the BGA scheme, and were not CPLs - to be a TMG examiner you need to be.
In practice it seems to work out OK - don't know where you are based, but Ray Brownrigg at Enstone (Oxford Sport Flying) is a good man and would be happy to sort you out or at least advise further.