I forgot to post about my SX-70 experiences.
Basically, one half photo came out. There's something wrong with the light proofing, the shutter, or something else. Plus I had to hold down a flap so the photo would come out. The best bit was lining some family up and popping a 1970's flash bar at them. the astonishingly bright flash left them with eye spots for a while, and the crinkly noise of the over-heated blue flash bulb was very evocative.
The SX-70 goes back to the museum shelf. And at over £2 a shot I have no intention of getting a refurb, or a new one.