mgclements,
It's your SEP rating that has expired, your licence is still valid & is now an EASA (A for aeroplane) licence. You can add a TMG rating to it by doing training & a skill test
but only at an RTF/ATO.
AFAIK there's only a couple of places in the UK that can do that, both in the Oxford area.
The NPPL(SLMG) route will still be available for another 10 & a bit months, the great advantage of this is that you will be able to do it through your BGA club. Para 3.2(b) on p14 of
this NPPL document explains what's required. An NPPL(SLMG) earned before 8 April '15 can be added to your PPL(A) licence as a TMG rating provided that you meet the requirements for the issue of a PPL, which you will do by virtue of your JAR SEP training. Section 4 Part P Page 14 (p514) of
CAP804 explains.
The other route is to do the necessary to get an EASA (S for sailplane) licence & add TMG privileges to it. You can fly TMGs & even add a tow rating to an EASA (S) licence.