I seem to have resolved this problem - sort of. I wondered if VLC might be struggling with the number of videos on my SD Card, so I moved some of them into a new folder on the SD card. When I went into preferences and selected that new folder they all appeared when I went back into the Video tab! So I tried to set up another folder and drag a few more in there - that worked, both folders selected in preferences and all now showing OK. In short, I ended up setting up 5 folders containing all the videos that had been in the original folder and they show up fine - the original folder, however, is still not showing, so I have now ditched it. So all now sorted but still none the wiser! Thanks for looking into this for me.
Pete