Nick H. It was in one of the CAA's documents but I can't remember which; it wasn't a recent publication. From memory for 'simple' types it wasn't necessary, for 'intermediate' it was recommended and for 'complex' it was pretty much mandatory (although the wording wasn't quite so clear cut but the CAA confirmed their expectations). I'm sure if you called the CAA SRG (if they still exist) they would confirm or otherwise.
To be honest I've been out of the loop for a while and the rules may have changed. The Australian CASA (CAA equivalent) came under some criticism (in respect of the absence of DO involvement) from an inquest following a fatal Strikemaster/JP accident in the outback a few years ago. It wouldn't surprise me if the rules had tightened somewhat.
EAP