This information is actually available on the CAA's website but I have just spent the last 30 mins or so waiting for the poxy pages to load (incredibly slow) so I got pissed off and gave up. There was a period before 1 Jan 2000 when, in order to align the renewal criteria, periodicity etc. all instructor ratings (whether restricted or not) were issued with 25 month validity. Now all renewals and issues are done using JAR criteria and validity periods.
If you can stand the waiting have a look on the CAA website for General Information Documents at
www.srg.caa.co.uk