Weird problem. I foolishly let the battery run completely flat and replaced it. The audio appears to work but there is no sound. Turn the volume control and nothing happens, no sound, no change on the display. Appears to switch between the modes OK but still no sound. Tried loading a CD. Loaded OK but no sound and wouldn't unload. Then the sound came to life for a traffic announcement and went again.
So the amp still works if you send a sound signal to it, but the head unit is only generating a sound output when the TA breaks in.
Any ideas anyone - they would be much appreciated. I checked the fuse (just in case) and that looks fine. As the car is a 59 plate with 175k on the clock I am not going to spend a lot of money on it. If the worst comes to the worst I'll put a battery powered Bluetooth speaker in the car, which I only expect to keep for a year or two.