Answering my own question. I wss under the false impression that I was the person who loaded the 1200 items into the Music Finder on my PSR 750. Not so!!!!!
The 1200 were put in there by the manufacturers, and it is impossible to accidentally remove them.
How ever, they can be removed one by one, and it is an easy procedure.
I just chose one that I thought I would probably never need, and hit delete. My PSR 750 immediately showed that I had only 1199 left. I now had room to put one in which I thought was likely to be used frequently by me. And that dreaded sign "Music Finder is full" did not appear! Phew! I now had 1200.
I doubt if it is possible to do a bulk delete- for example, as I only speak one language, it would be handy if I could delete all foreign titles in one hit. But maybe someone reading this will post in a "How To" from Yammyfan in Australia.