According to AMC1 FCL.625(c) , if the rating has expired by less than three months, no refresher training is required but you still need a certificate issued by an ATO to get the rating renewed. However, the stated opinion of the UK CAA (subject to approval by EASA) is that, within that three month period, the renewal may be certified by an IRE without reference to the Authority.
If the rating has expired by more than three months, there is mandatory refresher training, in accordance with AMC1 FCL.625(c), and the rating has to be re-issued by the Authority, for which they will require a fee. In the absence of any approved alternative means of compliance, the requirement is for 1 training session if the rating has expired by less than 1 year and 3 training sessions if the rating has expired by more than 1 but less than 7 years. 'Training session' has not been defined and , therefore, need not necessarily include any flying.
If the rating has expired by more than 7 years, the applicant is required to undergo the full training course for the issue of the IR (as well as re-taking the theoretical knowledge examinations).