I have done as you suggested then turned the camera on again. I operated the zoom in and out repeatedly for about twenty times when the message
'Turn camera off and back on again' showed. This cleared the problem but then it happened again when I repeated the zooming. It would probably take some time to find out if it will freeze completely. Do you want me to carry on doing this until the zoom freezes and I get the message 'System error (Zoom)' ? As I said it might take some time.
I did as before then put in a new SD card. The message 'Turn camera off and on again' came up after several zooming operations. I truned the camera off and on but the zoom then remained frozen with the image way out of focus. There was no 'System Error (Zoom)' message. After waiting some time to see if anything would happen I found I was able to manually push the lens back in, at which point the problem cleared. Normally the lens is locked in its frozen position and cannot be moved.
Many thanks for your help. As a 74 year old pensioner I can't really afford to
replace this camera at the moment so I will persevere with it for awhile then look for another. Thankfully it is only my back-up camera. Once again many thanks.