This particular series (i.e. the old one) is set in the twelfth century. The Friars weren't even established by that point, so Friar Tuck would not even have been a Mendicant Friar. Given the etymological roots of Friar (i.e.
frater or rather,
brother) it's no big deal, but suffice to say he wouldn't have been trained to combat heresy.
As to the Catholicism of England during the twelfth century, there would have been a fairly wide degree of variance within the standard. Of course, nothing that could be described as Druidic, but a fair bit that might be described as 'folkish'.
The series, however, clealry owed a lot to the reinvented Celtic movement and so it is not too surprising to find a fair bit of dodgy ideas about 'paganism'. It is worth noting that at the same time Kevin Costner was making his
Robin Hood: Prince of Thieves' version, there was another much better
Robin Hood television film released which owed a lot to the old series in look and style. Uma Thurman played Maid Marion.
[i]It is a joyful thing indeed to hold intimate converse with a man after one’s own heart, chatting without reserve about things of interest or the fleeting topics of the world; but such, alas, are few and far between.[/i]
– Yoshida Kenko (1283-1350), [i]Tsurezure-Gusa[/i] (1340)