So my details have been re-entered and I've given a new authorisation and I have another email confirming that the switch will happen quickly and easily and, this time, be complete by the 22nd November. I'll put it on the virtual mantelpiece next to the one confirming that the switch will be complete by the 1st November. This whole process began on 24th October.
One big attraction of this quick, easy switching process is that payments in and out (standing orders, etc.) are guaranteed to flow seamlessly from Old Bank to New Building Society for a year-plus while I get round to redirecting them. It hasn't occurred to me yet to worry about what might happen if those "systems go down" on a day when I'm expecting a payment - go down so badly that there's no evidence of the payment's existence let alone of the guarantee. But that kind of worry is not really a reason to keep a bank account open, is it?
I do wonder, though, whether those "systems went down" due to mass take-up of the opportunity to close bank accounts without hassle (sic). Crazy idea I know, totally silly, but...