The European Union legislation concerning compilation of Safety Data Sheets (SDSs) requires that the person compiling an SDS be "competent" (reference: REACH Regulation 1907/2006 Annex II Requirements for the compilation of Safety Data Sheets, para 0.2.3 of the introductory comments).
The Requirements then says "Suppliers of substances and mixtures shall ensure that such competent persons have received appropriate training, including refresher training."
This is basically the same as applied under the pre REACH provisions (E.g. CHIP3 and the L130 Approved Code of Practice: The compilation of safety data sheets).
There is no further official EU requirement or guidance as to what constitutes appropriate training/refresher training. There is no UK Government controlled, or approved, examination system with a set pass level and certification.
The requirement means that training should be given, or if applicable suitably refreshed, BEFORE a person is required by the employer to write or amend a Safety Data Sheet.
CHCS runs modular training for SDS writers, and the concept here is that the intended SDS compiler need only attend those modules of relevance - e.g. if the person is a chemist then there is no need to attend Module 2 - Chemistry for non-chemists. Also it may be that the company has a person responsible for certain aspects of the tasks involved in order to compile an SDS, e.g. a Dangerous Goods Safety Adviser (DGSA) providing the data for Section 14 may obviate the need for the SDS compiler to attend Module 8, Classification for Transport, and Module 10, Transport Labelling and Documentation.
The full list of Modules can be found on the website.
Unfortunately, with current demand, CHCS normally offers each Module only once a year, which might mean that a person could take up to a year to become "competent" enough to undertake SDS compilation. For that reason you might want to consider running some of the Modules in-house or utilising a suitable CHCS listed consultant to carry out initial training to an adequate level to meet the legal requirements, with later attendance at some of the Modules to supplement, reinforce, extend or refresh this initial competence.