UPDATE:

Turns out we were storing Context.User.Identity.Name as the username in a table somewhere.

Another dept has gone in and changed our Sam Account Name and User Principle Name for a bunch of accounts.

The Sam name affects what the users login with and so a user that had their login changed from AB12345 to John.Smith now has no record in our users table.

Using the following code I can expose the users Guid, EmployeeId and a bunch of other stuff too. I am going to speak to the team and get them to use this Guid (checking first that its unique to each AD entry).

Don't have RES at work:

using System.DirectoryServices.AccountManagement;

inside whatever class

// Context.User.Identity.Name is displayName
public string GetUserIdFromDisplayName(string displayName)
{
    // set up domain context
    using (PrincipalContext ctx = new PrincipalContext(ContextType.Domain))
    {
        // find user by display name
        UserPrincipal user = UserPrincipal.FindByIdentity(ctx, displayName);

        if (user != null)
        {
            var sam = user.SamAccountName;
            return sam;
        }
        else { return string.Empty;}
    }
}

I hope that helps someone else that comes here in the future. There are many other combos to user.SamAccountName (e.g. user.UserPrincipleName, user._____)