You wait ages to find one hoard of gold coins...

For 22 years, Dave Crisp had tramped out every week with his metal detector slung across his shoulder in the hope of finding a hoard of buried treasure that he promised his wife he would one day bring back home to her. After waiting for over two decades for the discovery that had so far eluded him, he stumbled across not just one rare Roman treasure trove of coins but two in the space of a single week.

His first discovery – 61 silver Roman coins scattered in a Somerset field, along with a child's bangle – was exciting enough after such a long wait. But it was easily eclipsed by his second – a pot of coins so extraordinary it is set to provide new insights into the history of Roman rule on the British Isles.

Yesterday, the British Museum announced the fruits of Mr Crisp's extraordinary second find: some 52,000 coins dating to the 3rd century AD – a period barely touched in most history books – uncovered one sunny afternoon in April.

The pot – a type of container normally used for storing food – contained a large number of coins from the reign of Carausius, who ruled Britain from AD286 to AD293 and was the first Roman to strike coins in Britain.

Mr Crisp, 63, a part-time hospital chef from Devizes in Wiltshire, said: "I couldn't believe it when I found the second hoard because when I found the first, I thought 'this is it!' The second discovery knocked the first one into touch. People go for 20 or 30 years never finding a hoard of coins. I found two in a week," he said.

On finding the second buried treasure in a field in Frome, Somerset, he said his detector emitted a "funny signal" which piqued his curiosity. He began to dig down and found 21 coins, followed by a 2ft high pot with 160kg (360lb) of coins stuffed inside.

Thrilled by the glimmer of the buried treasure, he phoned his local council who sent archaeologists to help with its excavation.

Roger Bland, the head of portable antiquities and treasure at the British Museum, said 766 of the coins found were from the reign of the "lost" British emperor Carausius – the largest amount ever to be found from this period – who ruled without the authority of Rome.

Carausius fell out of favour with the Roman emperor Maximian after he used his Channel fleet to amass enormous wealth by capturing pirate ships. Maximian ordered his execution, but Carausius, now regarded as a rebel, refused to submit and carried on ruling Britain and northern Gaul in defiance of Rome.

Before his eventual defeat, he became the first emperor to strike coins in Britain to give his reign legitimacy. Five of the Carausius coins are solid silver, the first such pure coins minted anywhere in the Roman empire in over 150 years. Most of the other coins are a relatively common denomination known as "radiates", made of debased silver and bronze.

Mr Bland said the hoard "has a huge amount to tell about the coinage and history of the period as we study over the next two years".

"This find presents us with an opportunity to put Carausius on the map. Schoolchildren across the country have been studying Roman Britain for decades, but are never taught about Carausius – our lost British emperor," he added.

A coroner is set to declare the find a treasure trove at an inquest on 22 July, paving the way for Somerset County Council to buy it from Mr Crisp and the landowner. Each is entitled to a 50 per cent share of its value under the Portable Antiquities Scheme. Its worth is not yet known.

Mr Crisp's discovery is only slightly smaller than the largest ever British Roman coin hoard, of 54,912 pieces, found in two pots near Marlborough, in 1978. A selection of the Frome coins, found in April, is to go on display at the British Museum from 22 July.

The largest ever hoard of Anglo-Saxon gold was found last year in Staffordshire in a farm field by Terry Herbert, an amateur metal detector who lived alone on disability benefit. It was valued at £3.3m, containing more than 5kg (11lb) of gold and 1.3kg (2.9lb) of silver.