And sorted - for £1.74!
It turned out, after a fair bit of trial and error, that it was the SD card slot software that was screwing everything up. Provided I didn't try to use that, the satnav, the iPod, the radio, the Bluetooth, the CD/DVD player, and probably the A2DP streaming from the iPhone, are all fine. I put the music on a USB stick, and it plays perfectly from there.
The £1.74? That's the cost of a USB extension cable from Fleabay, so I can keep the USB stick in the ashtray in case I want to get at it to add some more tracks.
Now I only need to keep my fingers crossed that nothing else goes wrong...