Your comments

The store just says that this device does not support the app - iOS 11 is required.

However, I managed to get it sorted. I used another computer and bought it on there. (It's still running iTunes version 12.6) I've just managed to install it on my iPad, using the download old version feature.

I wonder if you're able to provide a link / store option for the older version.

I do not have any devices running iOS11, and the new version of iTunes no longer supports the app store at all.

This effectively means that I cannot get the app at all :o(