Looks like we're going to get a new van and rather than trying to put some sort of soft fibre type insulation behing the ply lyning for heat/sound insulation I got to thinking of a few other alternatives.
Perhaps the easiest, Some form of multi-layered reflective membrane stuff like "tri-iso super 10" or more unusual, cavity foam insulation. Thoughts on the cavity foam would be I guess aproaching a company then seeing what could be aranged from there.
Heat radiated into the back of the van in summer would be one of the main reasons for insulating, but a well insulated van would make a good base for a camper conversion later on in the van's life.
Cheers Pete