Well, eight days later...
I've removed the whole dash, and replaced the O-rings. No leaks, and I've finally replaced the dash. It mostly lines up, although the rear right bolt securing the passenger airbag was bad to reach both in and out, and the driver's upper air vent was a pain to replace. Only one screw left over! I didn't refit the clips securing the air vents at the end of the dash, as I couldn't see how they clipped in, and the vents are quite secure anyway.
Replacing the SRS fuse has cleared the on-screen fault.
So far, so good.
But I've lost the right-hand-side speaker output so the radio unit will have to come out again :-(
And I've been experimenting with adding an AUX In to it, hooked up to a cheap Chines Bluetooth module. Perhaps it was my wiring/connections/soldering.
Whatever, the car's usable again. Only the air suspension to renew, and the tailgate bottom to fix.
Heigh-Ho.
PS I haven't worked full-time, just when it wasn't raining/cold/windy. I estimate it's taken me 12 hours all in.