Pass a skills test with a CAA rated TMG examiner
- or -
conversion training to an acceptable standard with an NPPL SLMG instructor, cheque to NPPL people for an NPPL with SLMG rating & then another cheque to the CAA to add a TMG rating to your PPL (assuming you have 75+ hours total time in SEP & TMG).
The latter sounds convoluted and expensive, but bear in mind that the training to safe standard should be the same in both cases and the CAA fees are (I think) the same either way. So either you pay a TMG examiner to test you or you pay the NPPL for a license you immediately convert to a TMG rating.
I think that pretty much all TMGs are taildraggers so, with the right instructor, you can add a tailwheel rating to your PPL at the same time.
sources:
LASORS F5
NPPL
cross crediting allowances 3.1