"Tess" was a recent great example of getting folk song into period TV. It worked for me. I think if a production is good enough, the odd bit of ahistoricalism (if that's a word) won't spoil it. I think the key is using folk music (or faux folk music in the case of the Wicker Man) as an integral part of the film rather than just an add on.

Unfortunately the only person I know who worked in TV gave it up for a job with the council...