500 Hour Criterion
I have a definitive answer on this now as per the CAA. It is not in the ANO or LASORs. However it is a requirement as per JAR-FCL 2.330(b).
The 250 requirement Whirly is a min TT expereince requirement to begin an FI(H) course. Once qualified you can only teach to PPL level until the restriction has been lifted and the above regulation is satisfied.
TT