Okay, so this afternoon I've finally got it off!
There was a trick to it, which is a different action to that shown in any of the youtube vids or discussed here, and I don't know if that's because it's such an early car.
Anyway, it was clear that no amount of pulling, or pushing down was going to work.
This only left a few directions and, with the screws removed and the side clips disengaged, I tried pushing the lower section in, away from me, toward the front of the car. There's about 1/8" movement in this direction if you push hard enough and, if you hold it in and gently move it down, then two hooks on the back disengage and the frame becomes loose in your hand.
We have lift off!!!
Just need to get the instruments out now, and apart, and find and fix those dry/cracked joints on the board.....