Please can you advise how to unlock my phone after having locked it with Android Device Manager as it was suspected a stolen. It has now been returned and I don't know how to unlock it.
Welcome to Android Central! When you locked the phone using ADM, I believe it asked you to create a new password. I hope you remembered it, because that's what you have to use to unlock it again. https://support.google.com/accounts/answer/6160500?hl=en
If you can't remember the lock code you used to lock it....just Use Android Device Manager, again, and relock with a password you know. Or if you've set up Samsung Find My Mobile, you can set a new lock code there as well.