One afternoon earlier this summer, in a Somerset meadow, David Crisp stumbled upon 52,000 Romano-British coins, the second-largest such hoard of its kind ever unearthed – and presently on exhibit in the British Museum. Almost 800 of these were minted during the reign of Carausius, which lasted from around AD286 until AD293, the first ruler since the conquest in AD43 to govern Britain without the authority of Rome – and a much-overlooked historical figure. As Roger Bland, the museum's head of portable antiquities, says, "This find presents us with the opportunity to put Carausius on the map. Schoolchildren across the country have been studying Roman Britain for decades, but have never been taught about Carausius – our lost emperor."

For nigh on 10 years prior to its recapture, Britain enjoyed the best of both worlds as a unified and isolationist nation-state that could still claim affinity with the greater dominion of Rome across the Straits of Dover. Indeed, some of the coins that activated Crisp's metal detector are embossed with the motif "AUGGG" (the three 'g's denoting three augusti, or Roman emperors), stressing that Carausius was on equal terms with the other two emperors – one in Constantinople, one in Rome itself – of an increasingly more fragmented federation, riven with incessant warfare.

Regarded, nevertheless, as a glorified squatter, Carausius was recognised only under official sufferance – and, with his passing, his rule was shrugged off in contemporary records as a triviality, just one of around 50 similar territorial uprisings. As a result, an important episode in our island story remained shrouded in distortion and obscurity. There's no mention of Carausius in the Anglo-Saxon Chronicle, and what might be described as the sole major biography is P H Webb's The Reign and Coinage of Carausius, which is 88 pages in length and was published in 1908.

The raw facts are that Mausaeus Carausius was born of a humble family dwelling in what was to smoulder into modern Belgium. On the strength of the coins he minted, he was burly, multi-chinned and unshaven – with the intimation that beneath layers of fat, the muscle was rock-hard, and that, if slighted, he could inflict red-fisted reprisal.

He began his working life as a common sailor, serving eventually as a steersman aboard a merchant vessel. His career took off when Rome, attempting to bolster its supremacy by employing the skills of conquered people, sought parochial bruisers to assist in the crushing of revolt in north-eastern Gaul. Emerging as a natural rabble-rouser, Carausius distinguished himself during these expeditions on land, but, a mariner by instinct, he was better placed to rise through the ranks of the newly reconstituted Classis Britannica – the British fleet – being promoted to high admiral after visiting Rome for what amounted to a job interview.

Unaware of any hidden agenda, the Senate commissioned Carausius to patrol the waters of northern Europe for buccaneers, mostly from Baltic and Scandinavian regions. Based in Gesoriacum (Boulogne), Carausius seemed to undertake this task with ruthless competence – according to Eutropius's Epitome of Roman History, penned a century later – until bureaucratic diligence brought to the ears of Rome that not only was he apprehending these nascent Vikings, but was appropriating their stolen goods. It was hinted that he was actually in league with them.

Maximianus, the latest in a line of emperors embroiled in a perpetual turmoil of back-stabbing – literally – and jockeying for position, suspecting the surfacing of a rebel or, worse, rival, made a cursory appeal to Carausius's honour by requesting him to report to the Senate for court martial and, in all likelihood, execution. Ignored, Maximianus then ordered an unsuccessful hunting-down of this bluffly popular naval commander, who, with the means to buy loyalty if necessary, withdrew the fleet into Britain and proclaimed himself the third emperor. Believing might was right and that the gods were on his side, Carausius made a stand behind the moat that was the misty Channel, and defied the continental Goliath.

The extent of the land's acceptance of the takeover is exemplified by the carving of "Carausius" on a tall milestone in distant Carlisle. Thus we had our own sovereign – albeit a probable hybrid of Al Capone, Idi Amin and Long John Silver – and a quasi-imperial insularity that obliged Rome to make a brittle peace, particularly after the defeat of a flotilla hastily assembled by Maximianus. Next came Carausius's subjugating tour of the north and an agreement with the Picts, who, vowing cheerful assistance against Rome, carried on as normal, looting, butchering and raping across the frontier marked by Hadrian's Wall.

More enduring was Carausius's creation of 10 forts from Brancaster to Porchester along the south-eastern coast. This came to be known as the Saxon Shore for the mercenary crews who, purportedly, earned a fearsome reputation in its taverns, and contained a preponderance of the first Germanic inhabitants of this sceptr'd isle. Ruffians they may have been, but here was a shadowy and guttural-accented link to what was to mutate into England – initially, seven kingdoms under a Carausius-like "bretwalda" – within five generations. The seed had taken hold, but the screams of the newborn Anglo-Saxon child were unimaginable when Carausius was short-listing candidates for what was a Chancellor of the Exchequer by any other name.

The ultimate choice was a certain Allectus, a model of quiet efficiency throughout the prosperity that characterised the Carausian usurpation. While most of the other bullion in the Somerset treasure – the equivalent of four years' pay for a legionary officer – was of the debased mixture of metals in use beyond Britain too, that of Carausius is pure silver. He and Allectus also struck bronze and gold. As much instruments of propaganda as currency, most bore symbols and slogans of triumph and bids for legitimacy. On some, the female figure of, presumably, Britannia, clasps a laureate Carausius by the hand. Others sport the legends "Genius Britanniae" ("Spirit of Britain") and "Restitutor Britanniae" ("Restorer of Britain"), in keeping with perceived native disenchantment with Roman rule. Later engravings even suggest Messianic qualities in Carausius – as instanced by "Expectate veni" ("Come, o long-awaited one") – and promise of not so much a rose-tinted future as the recovery of a wondrous past in "Redeunt Saturnia regna", abbreviated to "RSR" and meaning "The golden age is back!"

Polite society, if not the proletariat in Carausius's kingdom, would appreciate this as an explicit reference to lines from Virgil. High office had bred fine sensibilities, and Carausius appeared to be reneging on his upbringing by aligning with Roman rather than provincial culture, and styled himself as the imperial Marcus Aurelius Mausaeus Valerius Carausius. He also considered it politic to adopt the religion of the Eternal City – or at least, have Roman names for the deities that had informed his neo-barbarian youth.

To Allectus and other intimates, these new sophistications and pretensions were regrettable maybe, but harmless. Far more worrying was Carausius's growing certainty about everything he did and said, manifested most dangerously in his choice not to leave strategic initiative to his foes, but to front exploratory invasions of the mainland with eyes fixed on the ultimate prize of Rome. These proved to be exercises in futility, mainly because significant support could not be guaranteed, even within the area in and around Gesoriacum, the toehold on the continent – which was, in any case, to have its harbour blocked to Britain after a siege instigated by Constantius, Maximianus's junior partner.

Causing further anxiety was the non-commitment of scattered tribes and the incumbent and supposedly defected Roman militia. Despite regularly renewed treaties, the Pictish raiders were becoming bolder, and a vague but discernible general disorder and a constant flux of alliances and factions were fuelling the notion that Carausius, once the subject of terrified admiration, was not merely fallible but out of control.

Concentrating on the possible, Allectus arranged – through, possibly, doubtful collusion with Rome – the assassination of his master, shortly after the undermining loss of Gesoriacum. Assuming Carausius's titles and dignities – though content to be "AUG" rather than "AUGGG" on the coinage – the relatively uncharismatic Allectus was in command for only the three more years it took for the Empire to strike back.