May thanks Haratio for the reply. I have checked the fuse and it seems OK.
I quote a section a Dealers workshop manual for the radio in question :-
A system may be used to allow the set to be used without turning the vehicle ignition on (+ before ignition feed).
This function has a time limit of 10 minutes at the end of which period the set will automatically turn off to avoid discharging the battery.
5 seconds before this time period expires, a beep will be heard and the display flashes.
When + before ignition operation is interrupted by pressing button 1, pressing the same button again restarts the timed period of 10 minutes.”
This implies that it is one of the modes of the radio but does not state how to disable it. But I will check for loose connections in the morning just to make sure
Any thoughts would be gratefully accepted.