Why can't I change the 'sent messages' folder used by iPhone's Mail app?

I regularly switch between 4 devices: an iMac, a laptop (Macbook Air), an iPad and an iPhone. All four use the native Apple Mail client appropriate for their OS (up-to-date iOS for the iPad and iPhone, Mavericks for the Macs). My email is IMAP-based. The email account has been around long time and as a result the server setup these days is not the tidiest -- it has more than one 'Sent' folder ("Sent", "Sent messages", etc). This isn't a big issue as I can specify the correct one to use for 'sent' message inside each of the various Mail clients. Except, for some reason, the iPhone.

When I send an email from either my iMac, laptop, or iPad it appears in the normal 'Sent' folder -- in other words, the 'Sent folder at the top of the sidebar in OSX Mail. However, if I send one from the iPhone it does not. Instead, it appears in a different 'Sent Messages' folder listed down at the bottom of the sidebar (in OSX Mail) under that particular account's extra folders.

I've gone into the iPhone's settings (Settings > Mail, Contacts, Calendar > [the account's name] > Account > Advanced > Sent mailbox) and selected the folder that I want it to use ('Sent messages'); I've done this with the same settings open side-by-side on the iPad to ensure that I choose the right one, since the iPad's behaviour is correct. So I change the selection from 'Sent' to 'Sent messages'… backtrack through the screens, tap 'Done' when required, continue backtracking all the way out of Settings, send an email… and it still appears in the wrong folder. Returning to Settings it seems that the folder selection has reverted to 'Sent'. In other words, by previous change to the settings has been forgotten.

Anyone else have this? Is there a solution?

Thanks in advance.