This is cheeky:
Ratings check on a re-issued EASA Flight Crew Licence
Upon such application that would require an existing EASA Flight Crew Licence to be reissued, the applicant shall pay to the CAA a charge of £20 for all existing ratings associated with that licence to be checked for currency of validity.
Therefore to add an RT rating to a licence, it would actually cost you the extra £20 as well - so £99 rather than £79.