Environment version: 4.0.30319.42000
Json.NET version: Newtonsoft.Json, Version=11.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed

Result of deserializing and re-serializing the JSON: 
[
  {
    "pages": [
      {
        "FirstName": "Test1",
        "LastName": "Test2",
        "Address": "London, GB",
        "Error": "Something's gone wrong"
      },
      {
        "FirstName": "Test3",
        "LastName": "Test4",
        "Address": "NewYork, US",
        "Error": "Something's gone wrong"
      }
    ],
    "labels": [
      {
        "DisplayName": "ContactNumber",
        "Value": "01234 123 123"
      }
    ]
  }
]
Result is identical to desired JSON.