Here is Bede's own account of Raedwald:
"Reduald...ab uxore sua et quibusdam perversis doctoribus seductus est...ita ut in morem antiquorum Samaritanorum et Christo servire videretur, et diis quibus antea serviebat. Atque in eodem fano et altare haberet ad sacrificium Christi, et arulam ad victimas daemoniorum. Quod videlicet fanum rex eiusdem provincia Alduulf, qui nostra aetate fuit, usque ad suum tempus perdurasse, et se in pueritia vidisse testabatur."
("Redwald...was led astray by his wife and perverse advisors...so that, in the manner of the ancient Samaritans, he appeared to serve both Christ and the gods that he formerly served; and [so that] in one and the same temple he had both an altar for the rites of Christ, and a little altar for sacrifices of daemons. Which temple the king of the same province [Ease Anglia], Aldwulf, who lived in our time, said lasted into his own time, and that he had seen it in his childhood.")
This is the sort of politic flip-flop that a king, in a time of religious change, might find necessary for reasons of state. Bede's adding the source of his information might imply that he thought his readers would otherwise find the account incredible; which if true means that paganism had entirely disappeared by his time.
The other quotes kat provides illustrate some difficulties for studying the history of paganism. (1) Many people cannot separate religion from magic, while other people separate them quite easily; and (2) many Christians have had a tendency to fling the word "pagan" at other Christians whose customs they found strange or inappropriate.
The reports of people singing and dancing on Sunday tell us nothing about the religious opinions of those singers and dancers, though it tells us something about the moralist's opinions about proper behavior in church. The same applies to the report of people "conjuring the Rhine". We don't know, from that report, what those people thought they were doing, but analogous cases suggest that they considered themselves good Christians (see the "Shoney" practice referred to above.)
References by moralists to "Diana" are suspect. Diana is one of the Roman gods to be mentioned in the Latin Bible (Acts 19.28ff) so a moralist with a touch of scripture knowledge might not simply call people of whose customs he disapproved "pagan", he might go further and accuse them of worshipping "Diana". If he had a touch of classical learning he might accuse them of practising the "rites of Venus". Before I concluded that monks were actually worshipping Diana in woodland shrines, I would
(1) check the primary sources; the reports might contain less than meets the eye;
(2) survey the Corpus Inscriptionum Latinorum and the archaeological literature for Roman inscriptions to Diana in Britain, to see if, and where, this goddess was popular in Britain in pagan times. If she was little known, it is hard to imagine that a cult could "survive" where it never existed. If she was very popular, it would strengthen the case for such a survival.
(3) check the manuscript history of Catullus's poem Dianae sumus in fide/peullae et pueri integri. (One of the lovliest poems ever written in any language.) Though there are many motivations for studying and transmitting this poem, one supposes that if there were actual worshippers of Diana, they would have copied this poem if they had kown it.