From JAR-FCL1
(b) Renewal
(1) If an instrument rating, has expired, the applicant shall
(i) meet refresher training and additional requirements as determined
by the Authority, and
(ii) complete section 3b of
Appendix 3 to JAR-FCL 1.240 including the flight preparation as a skill test.
The rating will be valid from the date of
completion of the renewal requirements.
The CAAs requirement is in LASORS Section E1.5
RENEWAL OF AN IR(A)
Where an Instrument Rating has expired by more
than 5 years but less than 7 years and IR
privileges have not been exercised in any other
category of aircraft (eg helicopters) or under the
privileges of an ICAO licence, an IR Proficiency
Check with a CAA Staff Flight Examiner (for
single-pilot aircraft) or a Flight Operations Inspector
(for multi-pilot aircraft) will be required.
So you will have to do your renewal with a CAA FE.
If you go over 7 years you will have to sit the IR Theory Exams again!
Section 3b of Appendix 3 to JAR-FCL 1.240 is the
IR Test schedule