Ah, sorry I misunderstood from what I read in the link you shared.
Anyhow, I may have made a minor breakthrough. Your suggestion of taking an off-line copy of the db and doing a repair there with view to keeping a separate copy led me this post:
I followed the recommendations posted there to review / fix database errors and then put the resultant file back into Home Assistant. I’ve now run Purge and “hey presto”… my data didn’t disappear; but my database shrunk back down to the size it was a few weeks ago before it started its rapid inflation.
This may be a fix after all. Let’s see what happens after 4am tomorrow.
Thanks for all your suggestions.
I’ll share feedback tomorrow on whether the mystery if disappearing data has finally been resolved.