How would you get to it if you didn't know what it's title was?
The CAA website has a number of quicklinks. The one below lists the FEH and all Standards Documents:
List of Flight Crew Standards Documents
To renew an expired FI certificate, you will need to comply with FCL.940.FI:
(c) Renewal. If the FI certificate has lapsed, the applicant shall, within a period of 12 months before renewal:
(1) attend an instructor refresher seminar;
(2) pass an assessment of competence in accordance with FCL.935.
In summary: attend a FI seminar, training as required at an ATO approved for FI training (see StdsDoc31), plus a FI test (now called Assessment of Competence) with a
FIE.
ifitaint...