I held off on the reset option, so thanks for that info - obviously no point.
Fortunately, my LG TV gives me the option of accepting an update or keeping the current version of any given app, so I can afford to wait a bit and see how this pans out before chancing other app updates!
I suppose we will have to wait now for a working update if it is Amazon's "fault" so to speak.
Please let me know what response you get from Amazon, and I will likewise post anything I learn on here in the meantime (it is useful to know that the update is not working on 2 different LG TVs though as that may tend to indicate that it is indeed the app at fault).