Does fiction, or indeed faction, have to be accurate historically? Isn't entertainment the main point, perhaps even the only point.

No, it isn't! This is a real fallacy - just because most of us don't count the cuff-buttons when we watch Master and Commander, that doesn't mean you can ever say that "anything goes", in any genre. Even when they make up some daft bit of pseudo-science in Doctor Who, it sounds like a daft bit of pseudo-science - the Doctor doesn't wave a wand and say "Wingardium Leviosa"!

Here we've got a series ostensibly set in the early C20, just as Cranford was ostensibly set in early C19 and Dallas was ostensibly set in the 1980s. Flint tools would look wrong to all of us in any of those settings; jetpacks would look just as wrong. Wifty-wafty paganism doesn't look that wrong to most of us, so we don't care - but it does look wrong to people who know about the period, so they do.

If the BBC Bonnet Department had just wanted to make stuff up, they'd have prefaced Lark Rise with "A long time ago in a galaxy far, far away..."