So... I'm mobile again!
While I was adopting the T6 fusebox twisty posture again the missus was calling garages to see if anyone had someone who could look at our van AND be able to do it sooner than six weeks away. One of them, a VW specialist, recommended a chap called Craig who works under the name of
Lightrofit. He came round after his other jobs - I was encouraged to see he drove up in a Transporter with an Alpine headunit in it!
The dash fuse (top left on the fuse panel, 5A) was indeed intact, but he spotted that there was no voltage across it, so accessed the fusebox under the battery and there was a 30A fuse blown there which supplies the dash etc. It's not clear what caused that, though.
He checked the ECU for any unusual fault codes and that the battery was OK.
If he's on here - thanks again - you've saved us much grief juggling work, kids, etc - the last 24hours have been enormously stressful
Thanks also to you all who helped with suggestions. I've learned an awful lot.
AND that silvery USB cable has gone. I'm so chuffed now.
Cheers!