Paul,
1) get the SLMG rating,
2) get an EASA medical,
3) check that your language proficiency is at least level 4 & current,
4) fill out
SRG 1104, icluding both the SSEA & SLMG in section 5.
Do NOT apply for any sailplane licence directly to the CAA. You will need to take your licence rating revalidation page & your current medical, together with a photocopy of each, to a friendly RTF or ATO & get the head of training to certify that the copies are genuine before sending the copies to the CAA with SRG 1104.
You will need a separate sailplane licence to fly gliders without supervision. The recommendation is to apply through the BGA once the BGA become an ATO, (supposed to be happening this summer.) More
here. You can add the TMG to both the LAPL(A) & the LAPL(S).