CAP804 states the following:-
Renewal. If an IR(A) 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 Part-FCL; and
(2) complete a proficiency check in accordance with Appendix 9 to Part-FCL, in an aeroplane.
If the IR(A) 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.
Standards Document 14 specifies the suggested retraining which is:-
Expiry by less than three months - No retraining
Expiry between three months and a year - One training session
Expiry between one year and seven years - Three training sessions.