Thanks Simon, I expected as much, but obviously preferred to avoid reset if there was anything else I could try (particularly if someone else had already suffered the same problem).
Fortunately I do already use various sync/backups including Google+, my email uses IMAP so will automatically get sorted, so shouldn’t be too much of a problem.
I just need to thoroughly review everything on the phone and ensure I can backup everything really important, and restore where possible.
Thanks for the link to the Helium app. I’ll see what that covers and include that in the mix.
I’ll post back on how I get on and any issues.