Thanks for these further responses - am impressed by the way people have rallied round!
Where I am currently up to is that recovering the files is no longer a problem; it's fixing the corrupt ones. I've managed to recover most / all of the still images and it is now just the videos that are still a problem; they seem to be corrupt.
The various file recovery apps I have run have all brought about the same result with the videos ie they seemingly recover, but then when I try to play them they don't work. I get various results eg they won't even open and flash up 'This is not a type of file Quick Time player recognises', or they DO open but when I try and play the clip the window is blank and the slider moves across the bottom with a couple of seconds of recognisable sound, replaced by a loud and sometimes intermittent hissing noise. Or they open and you see a still picture, but when you hit play the picture remains frozen while the slider and sound behave as above.
Trying to open them in VLC doesn't improve matters, and neither does changing the file name or the extension.
So then I started googling 'recovering corrupted mov files' and so far have tried demo versions of about 10 different apps. Also I tried one called repair cycorder but you seemingly need a degree in computing to understand it and I couldn't get it to work! Most don't change anything BUT one possibly two DO seem to work at least on some of the videos. These are MP4 Repair and Treasured. They work not by you downloading an app onto your Mac but by you uploading the videos to their site, whereupon it 'processes' them, flashes up a price for the full thing and lets you download a 6-second clip to prove it works. The price, however is the major sticking point; for 1 video MP4 Repair quoted me US$47 while for 3 it went up to $84 - and I have about 100 of the damn things!
I was on the verge of giving up but now that I have seen the videos CAN be recovered it's given me more impetus to keep trying.
Anyone have any further ideas? Photographicsafaris - I hope your offer was genuine!!
Thanks in advance