This is to amplify what bingofuel stated earlier. From the AMC/GM to Part-FCL regarding refresher training for a lapsed class or type rating:
AMC1 FCL.740(b)(1) Validity and renewal of class and type ratings
RENEWAL OF CLASS AND TYPE RATINGS: REFRESHER TRAINING
(a) Paragraph (b)(1) of FCL.740 determines that if a class or type rating has lapsed, the applicant shall take refresher training at an ATO. The objective of the training is to reach the level of proficiency necessary to safely operate the relevant type or class of aircraft. The amount of refresher training needed should be determined on a case-by-case basis by the ATO, taking into account the following factors:
(1) the experience of the applicant. To determine this, the ATO should evaluate the pilot’s log book, and, if necessary, conduct a test in an FSTD;
(2) the complexity of the aircraft;
(3) the amount of time lapsed since the expiry of the validity period of the rating. The amount of training needed to reach the desired level of proficiency should increase with the time lapsed. In some cases, after evaluating the pilot, and when the time lapsed is very limited (less than 3 months), the ATO may even determine that no further refresher training is necessary. When determining the needs of the pilot, the following items can be taken into consideration:
(i) expiry shorter than 3 months: no supplementary requirements;
(ii) expiry longer than 3 months but shorter than 1 year: a minimum of two training sessions;
(iii) expiry longer than 1 year but shorter than 3 years: a minimum of three training sessions in which the most important malfunctions in the available systems are covered;
(iv) expiry longer than 3 years: the applicant should again undergo the training required for the initial issue of the rating or, in case of helicopter, the training required for the ‘additional type issue’, according to other valid ratings held.
(b) Once the ATO has determined the needs of the applicant, it should develop an individual training programme that should be based on the initial training for the issue of the rating and focus on the aspects where the applicant has shown the greatest needs.
(c) After successful completion of the training, the ATO should give a certificate, or other documental evidence that the training has been successfully achieved to the applicant, to be submitted to the competent authority when applying for the renewal. The certificate or documental evidence needs to contain a description of the training programme.
Additionally, the UK CAA has issued an Alternative Means of Compliance as follows:
Alt MoC1 FCL.740(b)(1) – Validity and renewal of class and type ratings
RENEWAL OF CLASS AND TYPE RATINGS: REFRESHER TRAINING
(a) Paragraph (b)(1) of FCL.740 determines that if a class or type rating has lapsed, the applicant shall take refresher training at an ATO. The objective of the training is to reach the level of proficiency necessary to safely operate the relevant type or class of aircraft. The amount of refresher training needed should be determined on a case-by-case basis by the ATO, taking into account the following factors:
(1) the experience of the applicant;
(2) the amount of time elapsed since the privileges of the rating were last used;
(3) the complexity of the aircraft;
(4) whether the applicant has a current rating on another aircraft type or class; and
(5) where considered necessary, the performance of the applicant during a
proficiency check for the rating in an FSTD or an aircraft of the relevant type or class.
It should be expected that the amount of training needed to reach the desired level of competence will increase with the time elapsed since the privileges of the rating were last used.
(b) Once the ATO has determined the needs of the applicant, it should develop an individual training programme based on the ATO’s approved course for the rating, focussing on the aspects where the applicant has shown the greatest needs. Theoretical knowledge instruction should be included as necessary; such as for type-specific system failures in complex aircraft. The performance of the applicant should be reviewed during the training and additional instruction provided where necessary to reach the standard required for the proficiency check.
(c) After successful completion of the training, the ATO should provide a training completion certificate to the applicant, describing the training provided. The training completion certificate should be presented to the Examiner prior to the Proficiency check. Following the successful renewal of the rating the completion certificate and examiner report form should be submitted to the competent authority, together with the relevant application form if the examiner cannot sign the certificate of revalidation in Section XII of the UK-issued licence.
Note: [..]
Finally, and with regard to the requirement or otherwise to hold a valid class or type rating to which an IR can be "attached", I am not aware that this is yet in force. However, NPA 2014-29A proposes the following with regard to the revalidation of the IR(A) (my underlining):
FCL.625.A IR(A) — Revalidation
(a) Revalidation. Applicants for the revalidation of an IR(A):
(1) when combined with the revalidation of a class or type rating, shall pass a proficiency check in accordance with Appendix 9 to this Part;
(2) when not combined with the revalidation of a class or type rating, shall:
(i) for single-pilot aeroplanes, complete section 3b and those parts of section 1 relevant to the intended flight, of the proficiency check prescribed in Appendix 9 to this Part; or
(ii) for multi-engine aeroplanes, complete section 6 of the proficiency check for single-pilot aeroplanes in accordance with Appendix 9 to this Part by sole reference to instruments.; and
(iii) hold the relevant valid class or type rating.
(3) An FNPT II or an FFS representing the relevant class or type of aeroplane may be used in the case of paragraph (2), but at least each alternate proficiency check for the revalidation of an IR(A) in these circumstances shall be performed in an aeroplane.
(b) [..]