Here is Ronald Hutton's summary of the "Diana" business in Devon, from The Pagan Religions of the Ancient British Isles, Blackwell, Paperback edition 1993, p. 299-300:
"[I]n 1351 the monks [of Fithelstock Priory] erected a chapel in a wood nearby, where thy installed an altar, a rack of candles and an image which the Bishop of Exeter described as being of 'proud and disobedient Eve or of unchaste Diana' rather than of the Virgin Mary. To this they attracted the local people, and made money out of them by reading their futures according to the casting of lots. The racket was broken up by the bishop, who had the chapel and its contents destroyed. The only other evidence we have that bears on the case is that the priory already had a bad reputation: in 1340 its sub-prior had to do penance for laziness and sexual misconduct. What is missing is any indication of the viewpoint of the monks themselves. It would be very interesting to know whether they were conducting a self-conscious parody of the Christian religion, or whether they were so ignorant and undisciplined that they genuinely did not realize that they were acting outside it. Whatever the truth of the matter, there is nothing in the story to indicate that they were acting in accordance with a local pre-Christian cult. Rather...they were deviants from medieval Christianity."
I agree with Hutton that it is most likely that the monks were spontaneous deviants, not bearers of an ancient non-Christian tradition. I note that it was the Bishop, not the monks (as far as we know) who identified their statue as "Eve or Diana". Without the image, we don't know if the bishop was reasonably shocked by the way the statue was carved, was overreacting to sculptural ineptitude, or whether the sculptor simply produced what we would see (even if the biship did not) as a portrayal of the Mother of Jesus as a strong, self-confident matron.
I must take Hutton's word for it that there is no evidence of Diana-worship in the same area in Roman times. If I ever want to check up on him myself, I'll take the route I mentioned earlier: search the Corpus Inscriptionum and the archaeological literature for evidence of a Diana cult.
Hutton's citations for this incident are:
W. H. Mandy, "An Incident at Bexley", Woolwich and District Antiquarian Society Annual Report and Transactions 1920-5, 23, pp. 25-37.
Jefffrey Burton Russel, Witchcraft in the Midddle Ages, Ithaca, [NY] 1972, p. 164.
R. P. Chope, "Frithelstock Priory", Report and Transactions of the Devonshire Association 1928, 61, pp. 175-176.
I have not seen these sources yet.