I can't update beyond iOS 10.2.1 (14D15)

I was originally enrolled on the Beta program, but since iOS 10.2.1 have been unable to get the update to verify; I've visited all the forums and tried absolutely everything. I tried pressing the power button on and off Held the power button to restart my iPhone and tried installing again Deleted the update, redownlaoded and tried installing it again Turned off wi-fi and tried to verify over 4G Gone into iTunes and tried to update there but it says iOS 10.2.1 is the latest version and there is no further update; contrary to what it says on my phone Tried to update iTunes and I says it's the latest version Have deleted the beta profile, gone on the website, unenrolled, deleted the update, hard reset my phone, and now update 10.3.1 is available, which downloads, but again won't verify. Have tried all the above again, including scheduling the update overnight, only to wake up to see the update has failed. Have tried everything, the updates all download but just WON'T VERIFY no matter what I do, and my iPhone 6+ is permanently stuck at 10.2.1 (14D15). I've got about 7GB space which should be more than enough. Visited all the forums and googled same problem and can't find a solution anywhere that will just get it to push through and verify so it will install the update. HELP!!!