What have the CAA told you?
Subpart J, FCL.900 states that a licence is required, which you have, but FCL.915 (b)(4) requires you to be entitled to act as PIC in the aircraft during flight instruction therefore, if your SEP has lapsed, you are unable to exercise the privileges for flight instruction until you renew your class rating and assuming a still valid FI rating.
The answer to your second question can be found in FCL.940: For an FI(A), 50 hours instruction within the rating validity period, and if teaching toward the IR, 10 hours toward the IR in the preceding 12 months of certificate expiry; instructor refresher training at an ATO or with the competent authority (seminar), or an assessment of competence within the 12 months preceding expiry of the certificate, remembering that an assessment of competence is necessary for each alternate revalidation. Alternatively, as mentioned above, you could revalidate/renew an FI certificate by refresher training and an AoC without the experience.
As an FI though, surely you must at least have been aware of the validity of your privileges?
Last edited by Reverserbucket; 20th Jan 2021 at 09:37.
Reason: Other responses have already answered since typing