OK. So after trawling the forum I've found that this is all to do with the date system that AM uses and that there is a workaround, but you have to recreate your mailbox from scratch...so a bit of a pain to reset all other bits that I'd done previously
So the headings are slightly different due to the latest version that I am using, but all found in the same areas. Below is the copy and paste from the other thread that I found the fix on:
Putting it all together:
1 - You will need to add your new, migrated, mail account into Aqua one more time. Before saving the account, do not enable IMAP push yet (even if you would like to use it). It's necessary to adjust a couple settings first.
2 - Back on the main screen, and without tapping the account's sync button, long press the account -> "Options and folders".
3 - Under "IMAP options", change "Which message date/time to download" from "Date/time received" to "Date/time sent".
4 - Back out to "options and folders", open "Account options", and change "Messages to sync" to "By time period", picking some number of days (7 or 10 would be a good start).
5 - Back out to the main screen, tap the account's refresh, then enable push mail if desired.