It's the within a period of 12 months before renewal bit I'm finding confusing
Within a period of 12 months you must attend a FI Seminar and pass an assessment of competence to Renew your FI Certificate. If its still currrent, you Revalidate it, if its expired you Renew it.
Do both need to be completed within a stipulated time frame?
Yes, 12 months.
Provided the Certificate is listed in Part XII of your licence, either the Seminar Provider or the FIE can Renew it, but if its on the back of your licence, only the CAA can renew it, and that process is included in the 12 months.