Unable to connect to imap email on Wifi with iOS 6.1.1

I am unable to get my imap email when connected to WiFi. It works fine on 3G. I'm using iOS 6.1.1 on an iPhone 4S and the issue seems to have started since updating the software. Have tried most things (reboot, uninstall and reinstall mail settings, reset newtork settings etc) but nothing seems to work. Currently the only way to check my email is to turn WiFi off!