Feel free to shoot me now in flames if you so wish but here is my understanding of the law.
Under the old UK licencing requirements the FI rating came back with restrictions (so to speak) on night flying, applied instrument and aerobatics. A course had to be completed to lift said restrictions - applied instrument requiring the 200hr IFR (50hr IF) and an unrestricted FI.
Under JAR FCL and CAP 393 - as the fancy green book isn't the ANO - the CAA no longer issue UK FI ratings - they are all JAR instructor ratings.
The JAA in their wisdom do not place a "no aerobatics" restriction on FI ratings - and as such one can teach aeros with no training in them at all.
I personally do not agree with the JAA standpoint or that the CAA have succombed to their preaching on such things but legally one can bugger about teaching Bloggs aerobatics whether we as instructors can tell our arse from our elbows or not on the subject.
The 01/2005 Trainingcom happend to bring our attention to the subject so we dug around through the books to study it in more depth - Lasors, JAR-FCL and CAP 393 all got a good looking at in the process.
I might add though that David Scouller is a good guy who knows his stuff and even if no longer a legal requirement getting the gen off one of the good guys is no bad thing - they'll try and kill you S&L let alone upside down and close to the stall.