Apparent cause of problem is the mess that happens to disks when many and large .avi files are moved about and worked with - as in the editiing process.
Lesson learnt and passed on; if working with such material, defragment disks often!!
A commercial product from the suggested Easeus orgainsaton did recover some files and got the disk cleaned, reformatted and running again but many of the .avi files, although recovered are corrupted beyond use.
Another two days lost to playing with computers rather than getting the job done....
sigh....