OK.
Amazon could not understand why I was being asked to pay for the free film.
As the only way I can view this film free is on my smartphone, I have been desperately trying to find out why I cannot cast the film from phone to TV.
My phone is Android 13 and normally, what I do (usually with a YT video) to watch it on the TV is to tap the cast button on the app and a search for devices commences. I then choose which device I want.
The cast button in Prime Video is not finding any devices.
I spent ages in a rabbit hole with the Samsung Smart things app because I discovered the TV has two old phones of mine listed and wanted to add my current phone in case that was a stumbling block, but to no avail.
So next I went to settings in the phone and pulled up CAST.
It found both the TV and the Roku, so I connected to the Roku first.
I found I could play the film, but here's the rub.
I could hear the audio and see the subtitles, but the screen was otherwise blank. I looked down at the phone screen and could see the picture, but not on the TV.
When the screen was mirroring my phone prior to playing the film, I could see the phone screen duplicated on the TV screen, so I am properly stumped as to why the audio and subtitles come through, but the picture does not.....
Just to make sure I wasn't going mad, I tried the same thing having disconnected from the Roku and connected direct to the TV. The result was the same.
I am thoroughly mystified!