Cannot update iPad 1 to iOS 4.3 or higher long time observer of answers first time asker of questions...

I don't post this here lightly and don't imagine that I haven't tried standard apple forums before coming here baffled. 

I've been sent a build of an app in Test Flight but I can't install it on my iPad 1 because:

"Your device is running 4.2 GM which is less than the required version (4.3) for this build. Please upgrade your iOS install..."

I'm fine with that but when I 'Check for Update' on iTunes a helpful box comes up and tells me:

"This version of the iPad software (4.2) is the current version."

Things I have tried:

Restoring ipad to factory settings.
Ensuring I have the latest version of iTunes.
Deleting iPad back up (iTunes>Preferences>Devices) incase of corruption.

I have just downloaded version 4.3.5 from a third party and pressed 'Check for Update' (while holding down the Alt key), accessed the file and tried to upgrade that way...

iTunes now tells me:

"The Device "#### iPad 1" could not be updated. This device isn't eligible for the latest build."

Help...
It won't be possible to updated to iOS 4.3 unless you have a software image for iOS 4.3 handy.  This is because Apple wants to force you to install the latest version which is iOS 5.0.  The test app you have been sent should still run on iOS 5.0 though.
Since iTunes isn't being nice about letting you upgrade, what you can do is go download the iOS 5.0 software image from Apple and then you can force iTunes to update using the image.  Here's what you do.

Download iOS 5.0 for your iPad from the following link (please be aware that this link is not a direct download from Apple.  Apple doesn't offer the ability to download iOS outside of iTunes which is why I've provided this link)
http://www.ijailbreak.com/ipad/download-ios-5-firmware/

Plug your iPad into iTunes.  Hold down the Option key on your keyboard and click the &quot;Check For Update&quot; button in iTunes.  An Open File dialog box will open.  Select the software image you downloaded in step #1.

iTunes should upgrade your iPad.