Hi Ian,
Depending on the amount of space between your ply walls and the outer layer of your van, depends on what you use, or in my case, how much of the same stuff you use.
Firstly do you know of the stuff they call "bubblewrap" but its covered in silver foil - it may be the thermawrap you're talking about. I got a roll of cheap stuff (B&Q) and roll of really good stuff:
http://www.campervanconversion.co.uk/sh ... insulation (I have a spare full roll from my conversion if you would like it, £25 and postage, and its yours)
I applied this against the outer skins (where the window recesses are)
Then I used the builders wall insulation known as Kingspan or Cellutex (yellow high density foam with silver foil on either side) for my van (connect) I only had room for 25mm deep pieces, you may be able to add more.
And then in the gaps I used shredded plastic bottle insulation (another B&Q buy)
Finally I put a second layer of the foil over it all, slightly smaller than the ply panel.
I hope this helps bud, I was the same as you, reading all the information and getting confused. So I just combined them all!