I'm so pleased to report and confirm that you were spot on @Latchmor.
When I first wet up MBS ages ago I just included my Record TV folder in my library, and never thought about it again.
I have removed the folder from the library and done some more recordings - result: no more poster jpg's.
In hindsight it all seems totally obvious.
@krustyreturns, I'll continue to monitor future recording in regard to reliability improvements to my recordings, and post any results. Thanks a heap for your time.