Now, as I want to reset my phone to factory settings I thought I would run a quick google search on it to see if I can bring it up again if I ever need it in the future. Or whether I should just remove apps manually so that I can keep the app around for when I may need it again.
After my search, I can confirm that the "Diagonsitics" app is available to bring up again using the instructions below on your phone.
Hopefully this helps you understand your possible phone problems further!
(*typical problem for any phone when the bottom of the phone where the charging port is has been worn down or moved a little from normal usage).
Windows Phone 7
Nokia Diagnostics app
Nokia Windows Phones 7 devices have a "hidden" diagnostic app. This app can be used to confirm that device functionality is present and working as expected, and is particularly useful for verifying whether "odd" results are due to your code or to information from the hardware.
The tools can be accessed for the first time by entering ##634# on the call screen/dialler. Subsequently the app is listed with all your others under the title Diagnostics. The app is (anecdotally) present on other devices from other manufacturers, but may require an alternative key combination.
There is also a video and a diagnosics tool available for Nokia Windows Phone 8 as well. Just check out their website: http://developer.nokia.com/community/wiki/Windows_Phone_Diagnostic_Tools