An update on my contact with Citi, which sounds much like Beta4me 's experience.
Initially Citi customer service just blew me off saying there was no problem.
After a month of trying to no avail, I gave them 7 days to rectify and copied the Citi email that deals with FOS action. I got a similar response from the Citi FOS rep who also quoted their illegal t&c regarding paying off lowest rate balance first.
Kept trying to contact the Citi FOS rep but they wouldn't return my calls or emails over another 7 days.
So I pulled the trigger and made a FOS complaint. About 4 hrs later FOS advised that they had passed my message on to Citi's FOS rep. About 30 mins after this Citi FOS rep emailed me 3 times seeking resolution, including an odd email retracting their earlier (incorrect) advice on how payments are applied and offering to re-credit the incorrectly charged interest.
+1 for the FOS.
But Citi have a few more things to do before I will be satisfied. A major one is confirmation of no adverse credit reporting arising from this incident.
Compensation would be in order too but its not my primary concern.