The content of most EASA regs is divided between 'hard' and 'soft' law. While the Interception of aircraft implementing rules (IR) are contained in 923/2012, the means of compliance with these rules are found in another document, the Acceptable Means of Compliance and Guidance Material, abbreviated AMC and GM. These can be found on the EASA website as Agency DECISIONS, on the bottom of this page under the catchy title of 'Annex to ED decision 2013/013/R'...
EASA - Acceptable Means of Compliance and Guidance Material
The document link is here....
http://easa.europa.eu/agency-measure...2013-013-R.pdf
Open and search for '11015' or 'Interception', your choice.
Do SERA AMC also have to be complied? Yes, as the ICAO text is now transcribed as mostly EU law and spread out under IR, CS (does not apply in this case)& AMC. GM is guidance only.