Entitled to free iMovie but can only get it by paying

I have an iPad Air 128GB with iOS 9.3.1 and according to this article, I should be entitled to a free version of iMovie:

https://support.apple.com/en-gb/HT202794

However, although I can download all the other apps mentioned; i.e.Pages, Numbers, Keynote, iTunes U and GarageBand, iMovie need to be paid for.

The iPad Air was not released until November 2013 so it's not as if I am misremembering how old my iPad is.

Has Apple changed the criteria or have I missed something?

Anybody know anything which might help?