If it's more than 5 years since the SEP Class Rating expired, once the renewal LST has succesfully been completed, all paperwork must be sent to the CAA at the address indicated on SRG\1119, together with the relevant fee. If less than 5 years, there is no fee to pay to the CAA.
The fee for such a 5 yr+ SEP Class Rating renewal is shown as £87 in the current CAA Scheme of Charges
http://www.caa.co.uk/docs/33/262PLSCorr.pdf 