I am very grateful to Nelz and purplepenguin for their kind advice which I pass on.
My BIOS decided to go into SAFE mode and would not save my changes to the default settings, which meant that it defaulted to booting only from the HD.
My entry to BIOS had been via ¨Del¨ from the splash screen.
The splash screen also offered F12, which I had always ignored since I did not wish for possible probs.
However, pressing F12 offers you a choice of booting from HD OR CD.
I chose CD and was able to run Memtest from LXF166 to show my RAM is fine.
Thank you, gents.