Treasure Act 1996
So what is the legal position over such a find?
Under the Treasure Act 1996 people are obliged to report finding of "objects of outstanding historical, archaeological or cultural importance" to their local coroner's office within 14 days.
The coroner will normally direct the find is taken to a local museum who in turn must notify the British Museum or the National Museum and Galleries of Wales.
An inquest will be automatically held by the coroner to decide whether the find is treasure.
If it is it belongs to the Crown; if not it will be returned to the finder or landowner.
A treasure valuation committee then decides the 'market value' of the find and any payment for it could be divided between the finder and the landowner.
Avoid damage
Dr Ticca Ogilvie, a conservator of antiquities from Bristol Museums and Art Gallery says it is important to get expert advice to avoid causing any unnecessary damage.
"Mr Allen did the right thing bringing the coins and pieces of pot in so soon," she told BBC News Online.
There is a risk of further deterioration to the coins once they are out of the ground so immediate "first aid" action is needed to stabilise them.
Once the coins are exposed to air and moisture they can rapidly start to deteriorate.
Conservation work is a balance between exposing more of the coin's surface detail, and keeping the coin chemically stable.
"What we are doing is buying time so the coins won't be damaged by the effects of salts reacting with moisture in the air," she said.
'Brown sauce'
In case you fancy DIY restoration stop and think first.
A quick trawl of the internet reveals many "solutions" that make experts like Dr Ogilvie cringe.
Thirty-odd-years-ago a well-known children's television programme suggested using brown sauce to clean-up coins.
It is really corrosive and can remove a thin layer of the metal which could destroy vital detail or start bronze disease which causes the metal to soften and turn green.
Once damaged, it loses both its monetary and historic value.
Dr Ogilvie says the first step the experts take depends on how the coins are found.
"If a coin is found in relatively dry soil don't wash it but put it put it in a small self-sealing bag with a sachet of dry silica gel.
"And if you find it in muddy conditions you might start by soaking the coins in 100% distilled water to remove the grime then progressively changing that water for an ever increasing percentage of alcohol.
"Finally once the coin is got to the stage of 100% alcohol, you have got most of the water out.
"Then it is a case of letting the alcohol evaporate until the coin is dry and stable."
To do the conservation work effectively you need a bright light - preferably one that doesn't heat up the coin.
"And a pair of magnifying goggles or glass with at least X10 magnification so you can really see what's going on."
'Labour of love'
The conservation process is a real labour of love. Dr Ogilvie spends an hour on each side of the coin using a variety of artists' brushes and cocktail sticks to remove the corrosion.
"It is down to experience what tools you use. For example a scalpel blade can be used to lift the layers of grime being careful not to dig into the coin's surface.
"It's a very fine line between how much you remove.
"For example on Mr Allen's coins there is evidence of silver deposits. It would be so easy to remove this accidentally if you weren't careful."
So why go through all this time-consuming process?
"Ultimately what we are doing is buying extra time which will allow the coins to be studied in another 200 years."
As for Mr Allen, he still can not believe he made the discovery in his back garden.
