I'm in a similar position to the original poster with the added complication that as a gliding instructor I plan to get an SPL (rather than a LAP(S)) to sit alongside my newly converted LAPL(A) which is intended to replace my NPPL (SSEA) with TMG rating once the postman delivers.
The accepted advice is to add a TMG rating to both / all licences and if you are having trouble finding an examiner, can I recommend that you speak to Ray Brownrigg at Oxfordshire Sport Flying at Enstone. I'm sure he can sort you out.
Happy Licence Hunting!
FA