Karl,
Yes. I choose not to point out the bit about the "Maintenance reset" in my post. :-)
I've reported the fault, steeling myself to the fact that the agent would have to, as we say in the south-west, "go swimming off Weston-super-Mare" (ie, go through the motions) and try yet another factory reset. Saving the recordings is a bit academic, as I also can't replay any of them, even the stored ones that we were able to replay prevoiusly. I suspect that the hard disc drive has become faulty.
Ayway, we've arranged an appointment for an engineer to visit, presumably to replace the box, on 11 March (the first one available, so delays appear to be the norm).
However, firstly I expect him to try to visit a nearby similar adress (I live at X XXXX Court, XXXXXXX XXXX Close, there is also a X XXXXXXX XXXX Close) in order to get himself back on schedule and generate an abortive visit charge: I'll be waiting outside to 'intercept' him if he does. I really wish that I didn't feel the need to have to do this.
Secondly, I also expect him to bring the cheaper non-recording DN370 version of the box, despite the fault reported.
But, hope for the best, plan for the worst. I'll keep you all posted.