OK - so i seem to have cracked it!

Used this in a .bat file
exiftool -P -overwrite_original_in_place -r -ext cr2 -ext jpg -iptc:caption-abstract^<filename d:\testX >line1o.txt
exiftool -if "not $DateTimeOriginal" -filename^<FileModifyDate -d %%Y_%%m_%%d_%%H%%M%%%%-2c.%%%%e -r -P -overwrite_original -ext jpg -ext cr2 d:\testX >line2o.txt
exiftool -if "$DateTimeOriginal" -filename^<DateTimeOriginal -d %%Y_%%m_%%d_%%H%%M%%%%-2c.%%%%e -r -P -overwrite_original -ext jpg -ext cr2 d:\testX >line3o.txt
exiftool -if "not $DateTimeOriginal" -EXIF:DateTimeOriginal^<File:FileModifyDate -r -P -overwrite_original -ext jpg -ext cr2 d:\testX >line4o.txt
This is NOT elegant

, however it does this :- (for all files with ext JPG or CR2, in directory D:\testX and all its subdirectories)
line 1, Move current Filename into the IPTC field Caption-Abstract. This is for future reference, and is neat, because it appears in the Windows Explorer column 'Title' and can be viewed/added in the Details view of any Windows explorer page.
line 2, If there isnt a DateTimeOriginal (eg from scans or old jpgs) then set the Filename to be 2011_12_28_1706-cc.ext from the FileModifyDate (which is the best approximation to a useful date for the image)
line 3, If there IS a DateTimeOriginal, then set the Filename to be 2011_12_28_1706-cc.ext from theDateTimeOriginal (which is when the image was shot)
line 4, If there isnt a DateTimeOriginal, then set it to the FileDateModify (so now all my images have a 'Date Taken', visible in Windows explorer)
NB I output the results into a separate txt for for each line - eg line1o.txt
NB I haven't used the seconds part of the date for file naming, but i do have a 2 digit number if there are more than one image in a minute.
NB Now I'm hgetting the hang of this, I'm going to add lines to move the files into a 2011/2011_01_Jan type DIR structure!

(50Gb of images from about 10+ years)
Thanks to all on this forum and on the ExifTool forum at QueensU for helping!
I do think that a really good GUI on front of this tool would get easy backing from VCs in SanFran becuase there is a HUGE need for this as the years go on, and Phil Harvey, you could make a lot of money.
J