Those emails that are not arriving, are they in folders at blueyonder.co.uk?
If so, click on the three dots at the bottom of the screen and you'll see an option called Folders. Tapping on it will bring to all folders that are auto-synced. To add more folders, just tap "show all folders", choose the folder you want and tap "sync this folder".
Unfortunately the current email version in Win8.1 does not fire notifications about emails in folders.