I went to my bank, and traced through the relevant statements with them, and so far as they were concerned all the payments had been made - even the one that might have been affected by their idiotic suspension of my account (of which more, no doubt). Fair enough, if their records couldn't spot the problem, it was time to go back to RW and ask them precisely which payment had been defaulted, and when, and why, etceteree etceteraa.
I just called them. Bewilderment on the part of the young woman to whom I spoke, because she couldn't trace the problem. A few minutes passed, during which she presumably spoke to her supervisor, and then she returned with contrite apologies. All my payments have indeed been made on time, and it's a mystery to them why I should have received any such letter, or why the person to whom I spoke last week should have been definite that there was an outstanding payment.
*shakes heads* And these are the people who can write DESPITE THIS WE HAVE NOT RECEIVED ALL OR ANY OF THE INSTALMENT JUST DUE, followed by If we do not receive your arrears by return further action will be taken, people who can haul my arse into court for nonpayment. People whose bloody system, apparently, is inadequate to keep track of those payments.
Gah. And now I don't have the fun of shouting at my bank either.
So back to doing the mountains of everything else that need to be done :(