Hey all,
I'm sorry I didn't update this sooner.
So, I took the unit out. Single DIN affair, popped out like a piece of cake with the removal pins. Unclipped the connectors, and took it into the house.
A handful of small torx screws and I had the unit apart, I had to remove the face of the head unit for clearance but it was obviously simple to do. I could then get to the SD card. I removed the (suspected corrupt) 8gb card, and replaced it with a 64gb one. Took half an hour at most and that was being slow and careful and stopping to check photos online.
Back on with the cover, and unit back in car (refitting was as easy as removal - 1 minute job).
Inserted the Pug dealer firmware CD, and it all just jumped into life!!! The screen popped up, with an image the Peugeot Lion, and a message similar to "Installing software, don't press anything and keep engine running".
This took about an hour, but it got there, and upon completion, the whole infotainment system (and previously dead screen) was working again, as were the parking sensors, steering wheel controls, and whatever else stopped working when it went wrong.
Noticed it found satellites but had no maps installed, so I got the latest 2024 Western Europe maps for her, (from ebay!) which again installed very easily from a USB.
All is well that ends well. Dealer quoted her 3500 euros for this job from memory, and it cost me £55* for the SD card, removal pins, and software. So pleased to have saved her this money and I know it was very appreciated.
* I'm not including my cheap flights in that cost, because ultimately I had 4 days in the sun, in a lovely villa with great company
Dealers should hang their heads in shame....