Upon testing this, indeed it didn't turn on. I was ready to pull it apart and start probing the circuit, when my son started "playing" with it, and somehow it came on. Turned out it was just the power button that was faulty.
On further testing though, there were more faults due to corrosion (which had also eaten the power ribbon cable): the digitiser (the touch screen) was missing a column of input (where there was a big dent in the display) and the USB-C port had corrosion but also broken pins inside.
I replaced the digitiser, the power ribbon cable and (after many hours struggling with solder in tiny holes) replaced the USB-C port. All good again.