Thanks very much for that link, QuickTimeKirk. Why the .,DS files suddenly became visible I am not sure. Though I had been tinkering with Finder preferences and Terminal commands to try to make Finder and Spotlight find some kinds of file that they do not like by nature, I did not do anything as far as I can see to make invisibles visible.
I hope I am right in assuming that by using the command 'defaults write com.apple.finder AppleShowAllFiles FALSE && killall Finder' to make the .DS files invisible again, I have not done something else awful! I also deleted one or two of the .DS files before deciding to ask for help here.