Hi Chriskit
I used Yahoo Mail before Bt dumped them for BtMail.
In my time of using Yahoo Mail I found that for some reason iOS just simply does not get on with it. The main issue I had was my iPhone asking to confirm the password which i knew was correct and syncing issues.
After performing a Google search I found hits that also talked about Yahoo Mail not integrating well with iOS too.
You could send feedback to Apple via
https://www.apple.com/feedback/Possibly try another email App too. MyMail is a good sound alternative which I used to cure my issue with Yahoo Mail.