I must say that my information was hearsay and I can't find anything to substantiate it either.
I was under the impression that a CRI could train licence holders for additional ratings
No, wrong impression.
FCL.905.CRI CRI — Privileges and conditions
(a) The privileges of a CRI are to instruct for:
(1) the issue, revalidation or renewal of a class or type rating for non-complex non-high performance single-pilot aeroplanes, when the privileges sought by the applicant are to fly in single-pilot operations;
(2) a towing or aerobatic rating for the aeroplane category, provided the CRI holds the relevant rating and has demonstrated the ability to instruct for that rating to an FI qualified in accordance with FCL.905.FI(i).
No Night Ratings