[Cameronpaterson]

Hello - the news for me isn't good I'm afraid. Lessthanyouthink's advice was well thought through, but unfortunately based on the assumption that I didn't already have the app set to 'store all'. It turned out, when I checked the relevant app panel within the iOS System Settings, that I did. (I also did, I must admit, initially confuse this setting with the library download options - doh! Hence my possibly odd seeming questions earlier in this thread.) Following Sharkus suggestion to try switching the value, I changed the setting to permanently store 5, cleared my local data, and redownloaded the five most recently read books, including the problematic 'Vanished Kingdoms'. This morning, I opened the Kobo app on my iPad, read Vanished Kingdoms for a bit, closed it, and it immediately began to spontaneously download yet again! I also noticed that the app appeared to have reset itself to the 'permanently store all' setting.
Since I am assured that there no bugs, glitches or size issues within the app which could be causing this problem, the only remaining conclusion must be that there is some corruption or damage to the original source file for 'Vanished Kingdoms' on your servers and that is what is causing this persistent misbehaviour. That would also explain why this issue only began after I bought 'Vanished Kingdoms'.
So I have now emailed Customer Services asking for a refund on the book again. Let's hope they give it to me this time, instead of making mysterious references to new versions which have yet to appear.