Thanks for the thoughts.
My disks are all NTFS but the file sizes go up to 18GB. These are the original edits from which I would then make either MPEGs or DVDs.
The importance of defragging was made clear to me by a colleague of much greater avi experience and for sure, looking at the pre-defragging analysis, it is quite clear that such files make a complete dogs dinner of the idea of contigious files.
Also perhaps interesting to note that Norton Speed Disk appears to make a far more serious job of sorting the data and that the end analysis looks far more 'sorted' than managed by the Windows defrag jobby...
We live and learn....
jg