Many thanks for those welcome suggestions.
1 "Easiest way is to just install adobe reader and use that"
I have had the free Adobe Acrobat Reader right from when the iMac was new.
BUT you cannot make a change to a pdf document, such as rotating the view, and then SAVE it.
To make a SAVE you need a paid-for Adobe product, not the Reader.
With Preview you can ROTATE and then SAVE a pdf document.
2 "Have you tried using "sudo" in a terminal to rename the application? What about rebooting and dragging it to trash before doing anything else? Does that not work?"
No. Letting me loose on 'sudo' is like giving a toddler a loaded firearm to play with.
I don't know enough about sudo/Terminal to try this approach.
3 "10.2.8combo http://support.apple.com/kb/DL645"
That was really informative - I had no idea that such a download was available.
Unfortunately it only upgrades OS to Mac OS X 10.2.8 (6R73) which is the version I already have.
There is apparently no way of seeing whether or not Preview is contained in that Combo Download, so I have decided to leave it alone till I get into calmer waters.
I may then try it as it may contain other worthwhile changes such as better security.
NOW for the good news.
After messing around trying to drag PREVIEW.app files from LaCie back up drive, and getting nowhere, I gave up and used SHUTDOWN [normally the iMac is just put to SLEEP overnight].
Next morning, re-start and PREVIEW 2.0.1 (v201) is there in APPLICATIONS and working normally, opening jpg and pdf files just as it did originally.
I have no idea what was wrong or why it now works - just grateful for small mercies.
Thanks for the help.