As nick14 has indicated, I'm not sure there is anything in the Aircrew Regulation concerning the pre-requisites of a Senior Examiner. Part-FCL makes only a passing reference to senior examiners as follows:
FCL.1020 Examiners assessment of competence
Applicants for an examiner certificate shall demonstrate their competence to an inspector from the competent authority or a senior examiner specifically authorised to do so by the competent authority responsible for the examiner’s certificate through the conduct of a skill test, proficiency check or assessment of competence in the examiner role for which privileges are sought, including briefing, conduct of the skill test, proficiency check or assessment of competence, and assessment of the person to whom the test, check or assessment is given, debriefing and recording documentation.
There is also a similar mention in FCL-1025(b)(3).
The AMC and GM to Part-FCL is more enlightening, specifically AMC1 FCL.1020and FCL.1025 which expands on the qualification of Senior Examiners as follows:
AMC1 FCL.1020; FCL.1025
QUALIFICATION OF SENIOR EXAMINERS
(a) A senior examiner specifically tasked by the competent authority to observe skill tests or proficiency checks for the revalidation of examiner certificates should:
(1) hold a valid or current examiner certificate appropriate to the privileges being given;
(2) have examiner experience level acceptable to the competent authority;
(3) have conducted a number of skill tests or proficiency checks as a Part-FCL examiner.
(b) The competent authority may conduct a pre-assessment of the applicant or candidate carrying out a skill test and proficiency check under supervision of an inspector of the competent authority.
(c) Applicants should be required to attend a senior examiner briefing, course or seminar arranged by the competent authority. Content and duration will be determined by the competent authority and should include:
(1) pre-course self-study;
(2) legislation;
(3) the role of the senior examiner;
(4) an examiner assessment;
(5) national administrative requirements.
(d) The validity of the authorisation should not exceed the validity of the examiners certificate, and in any case should not exceed 3 years. The authorisation may be revalidated in accordance with procedures established by the competent authority.
As you can see from the above, national authorities are free to set the experience levels.
Finally, you may wish, however, to see the UK CAA's
view on this.