Well, thank you Diane and Andy, I think I am making progress.
Andy, my Preview version is 5.5.3
I decided to look through my HD and see what is currently in there. (I have two external discs where I keep photos, family trees and similar long-term stuff.)
I found I could immediately remove quite a few items e.g.
Stuff from my old Mac (2005) – Power PC e.g Netscape ? and Mac Janitor ?? etc
Tried to get rid of "Face Time" but it said "can't be deleted, required by MacOS X" (This must be a downright lie). How do I override the refusal ?
Various other weird items like Omni-Outliner (2009), ooVoo (very weird.) and so on
I then went to Disk Utility, checked the Hard Disc (OK) and corrected Permissions (quite a few)
All that took some time but I did a restart and we were up and running after 1 minute 45 secs.
Next: I see I have a very long list of "Utilities" about 40 ! However most are relatively recent (from 2011 to present), but there are a few going back to 2003 and some odd but probably useless items such as Desktop (Mac OS 9), Dossier Textbridge 2004, Making music 2003, User Guides and Information 2007, and several others of the same vintage.
Have you any other suggestions as to where to look for other items to put in the trash ?
Finally, You will be happy to know that I have not found anything dating from my first Mac 512k with external HD of 80k dated 1985. It is sleeping in a cupboard, still in its carrying hbag/harness which made it a portable at the time.