I didn't realise you could read them with a text editor, I assumed they were some form of code. Which is seems many of those I downloaded are.
I also think I now realise why they don't work. In many cases the directory path from the original location is set within the vismat. So naturally when I try to use them the location is different and the vismat or Vray can't find the relevant bitmaps.
Luckily many of these textures include the necessary bitmaps, so it seems as I use them I'll eventually build my own library.
Thanks again for your help Andy,
Regards
Joe.