First, 3rd century refers to anytime between AD 200 - 299. Britain was conquered by Rome in AD 43 but was taken by the Caledonians in AD 208 (early 3rd century). In AD 258 Postumus establishes his empire in Gaul and in Britain (mid 3rd century). However, in AD 274 both Gaul and Britain were recovered by Rome under Aurelian; but turmoil ensued for more than a decade after his murder (late 3rd century). So, "some disaster looming" in Hertfordshire is not a great mystery or puzzle to figure out. Now paganism in Britain was the norm until post AD 315 (early 4th century) "the edict of Milan" by Constantine,; and the edict against paganism published in AD 391 (late fourth century). Christianity did not take root in Great Britain until the 5th-6th centuries.