Update on this..
[Nationwide unwilling/'unable' to accept Standing Order payments to Nationwide credit card account from FlexAccount]After complaining to Nationwide (whose customer service numpty got completely the wrong end of the stick, replying "We'll pass this on to the
*website* developers" when the problem is not intrinsically with their online banking but with their operational practice as a whole, which appears to disallow these transactions), I decided to give up.. :-(I then set up standing orders into my other current account, and then standing orders from my other current account to pay my credit card account. Cumbersome, but it does work!
..and then a couple of days later, one (but not both) of my botched (by NW) standing order attempts (which they transmogrified into a bill payment arrangement) mysteriously did re-transmogrify itself into a standing order after all..(!)
Ho-hum..