Regulation 1178
FCL.625 IR — Validity, revalidation and renewal
(d) If the IR has not been revalidated or renewed within the preceding 7 years, the holder will be required to pass again the IR theoretical knowledge examination and skill test.
It is an ATO who will tell you what you have to do, not the IAA or CAA! They are all required to comply with the regulation. There is no such thing as a frozen ATPL!
(c) Renewal. If an IR has expired, in order to renew their privileges applicants shall: (1) go through refresher training at an ATO to reach the level of proficiency needed to pass the instrument element of the skill test in accordance with Appendix 9 to this Part; and (2) complete a proficiency check in accordance with Appendix 9 to this Part, in the relevant aircraft category.