From Edward Siecienski's new book on the Filioque.
"Among the seventh century councils that taught the double procession, the Council of Hatfield (680) is perhaps the most interesting, especially as its president was a Greek, Theodore of Tarsus (602-90).
"According to Bede, when Pope Vitalian (657-72) appointed Theodore to Canterbury, he sent the monk Hadrian with him 'to prevent Theodore from introducing into the Church over which he presided any Greek customs'. Despite these misgivings, Theodore was a sound choice who antimonothelite credentials were impeccable (he had probably been at the Lateran Synod with Maximus the Confessor in 649). AlthoughBede claimed that Theodore convoked the Council of Hatfield 'to preserve the churches from the heresy of Eutyches', there is some evidence that he also used the gathering to respond to Bishop Wilfrid of York, who had been in Rome complainingabout Theodore's governance of the English Church. The Council's statement of faith, which apparently assured Pope Agatho of Theodore's orthodoxy, affirmed the faith of Nicea, Constantinople, Ephesus, Chalcedon, Constantinople II, and the Lateran Suynod of 649, and included belief in 'the Holy Spirit, ineffably proceeding from the Father and the Son, as proclaimed by all whom we have mentioned above, holy apostles, and prophets, and doctors.
"The two questions raised by this confession of faith were how Theodore would have interpreted this teaching, and how long the filioque had been part of the creed in England. While it is possible that Augustine of Canterbury (d609) might have taught the filioque during his mission to England (given his connection with Pope Gregory I), there is also a chance that it was introduced by Theodore's companion Hadrian, an African by birth whose study of Augustine and Fulgentius would likely have included their teaching on the procession. As for Theodore himself, it is possible that he understood the filioque in accordance with the principles Maximus had enunciated years earlier in the Letter to Marinus, especially if (as is likely) the two knew each other in Rome. What is clear is that Pope Agatho, although busy preparing his own statement of faith for the Constantinopolitans (without the filioque), happily received the proceedings of Hatfield, including its confession in the procession of the Holy Spirit from both the Father and the Son."
I wonder how those 'English Orthodox' and 'British Orthodox' who tell us that the Anglo-Saxon Church was "Orthodox", deal with the little matter of Hatfield.