try completely deleting the NTL email account from phone, turn phone off then back on, re-enter NTL email account info, i can`t garentee that this will work but it has been reported to fix most email problems. If you sync your NTL email with either Outlook or Outlook Express then you could also try connecting your phone to itunes, goto to the iphone`s info tab, scroll all the way down to email accounts, remove check mark from the NTL account, sync phone, replace check mark, sync phone (this will remove the NTL email account from phone then put it back on again).
Also are your O2 APN settings correct,
For Pay & Go customers the Celluar Data APN should be payandgo.o2.co.uk & MMS APN should also be payandgo.o2.co.uk
For those that are on contract the Celluar Data APN should be mobile.o2.co.uk & MMS APN should be wap.o2.co.uk
if your lost of email started after the CARRIER update then i would check to make sure that the APN settings are correct.