Why does virgin IMAP email keep failing

My wife and I both have iPhone 4’s and iPad 2’s, and all are set up to access our Virgin email accounts via IMAP using the imap.virginmedia.com server. For some reason my wife’s iPhone keeps having an issue every few days or so where emails cannot be downloaded and she gets a message saying ‘The mail server “imap.virginmedia.com” is not responding. Verify that you have entered the correct account info in Mail settings’.

In the past when this came up I used to close down the mail application and it would then work again, however since iOS 6 this does not fix the issue, and the only way I can get her email to work again is to delete the email account and rebuild it once more.

I am puzzled not only as to what is happening, but also why this only affects the mail on her iPhone and not iPad, and why it never affects me? I would normally think it is an issue with the virgin mail servers but as this only affects her iPhone (and not her iPad with the same settings) I am not so sure and so think it is an Apple issue. Can anyone help or have any suggestions?

Just to be clear, all IMAP accounts on the 4 devices are set up the same, and her email does work with the settings used for a time. It just keeps encountering this error on the iPhone (more frequently over the past few weeks).