Success! Joy Unbounded! Radio and CD both working again. Thanks to all for advice.
In the end it was intuitive, almost helpful, so long as I had the right code. Here is what I did.
Re-connected battery earth lead. Started engine. Radio came to life, displaying
---- 10
SAFE
I switched off the radio. Switched off the engine.
Ignition on 2
Pressing and holding in AS, switched on radio on/off button, radio obligingly beeped,and displayed ---- 10 , SAFE beneath.
I pressed button 1 4 times, display went to 1111.
Turned off ignition, paused, started again.
This time I pressed buttons 4 0 5 3 and 4053 appeared on the display;
the radio set began to seek radio stations, eventually playing Classic FM, until a government message broke in, ordering me to 'stay home unless absolutely necessary, then only to buy food, for exercise, or go to work, if working from home is impossible.'
I still had to put in the CD code. I pressed the CD button; the CD player promptly took over, playing disc 1, track 1, without asking for the CD code, which I never told it.
So, all's well that ends well. The car that I had planned to scrap, when its MOT expires on April 24th, lives on for 6 more months. In that period much may happen, something must change. I still think Sweden has it right.