Update to update: Was told to expect a 9.30AM phonecall from ATOS's audio recorder-wallah.
at 10.11AM I started phoning them on the direct-line number I was given yesterday. After 5 attempts I finally got through, and guess what? That's right, they
couldn't record the assessment today.
What the (flu-ey) young lady said was that if
she made me a new appt, it wouldn't affect my benefits (apparently, if "we, the claimants" want to change an appt we get ONE go, ATOS give themselves as many as necessary!), and because it was her as recorder-wallah making the appt for me, rather than ATOS making an appt THEN contacting her to see whether she could fit a recording into her schedule, the recording was a lot more likely to actually happen.
I asked (as usual) for written confirmation, and accepted a new appt for the middle of October.