The major Roman exhibition now running at Norwich Castle includes many finds discovered by local metal detector users.

The figures are astonishing. A third of all portable antiquities discovered in the whole country in any one year are found in just one county: Norfolk. 'And 10% of those are treasure cases. It's a staggering figure,' says Dr Tim Pestell, Curator of Archaeology of Norwich Castle Museum.

'Annually we record more than 20,000 finds in Norfolk. In 2012 there were 123 treasure cases in the county – a new British record – and last year 106.'

It's a glowing testament not only to our amazing heritage but also to the warm working relationship which has existed for many years between the experts at Norfolk's Historic Environment Service, Norfolk Museums Service and local metal detector users.

They share a love of the past which has produced many superb finds, many of which are being featured in the Roman Empire: Power and People exhibition, running at the castle until April 27.

When someone brings in a find for the experts to look at, it's dealt with by the Finds Identification and Recording Service. In Norfolk this is led by Dr Andrew Rogerson, with the help of coin specialist Dr Adrian Marsden, Erica Darch (who deals with most Treasure cases), plus Steven Ashley and Dr Mary Chester-Kadwell.

Most finds are not Treasure and can simply be recorded and returned to the finder.

The experts work within the framework of the Treasure Act, which came into force in 1997 and aims to be fair to finders, landowners and museums. Finders are given incentives to make sure they go about things in the right way. Crucially, finders can claim a full reward without having to actually dig up what they've discovered.

Say you've taken your metal detector out (with permission) and – lucky you – found a few gold- or silver-looking coins. Here's what you need to do.

The first thing to note is that you are legally obliged to report them within 14 days. If in doubt, always report a find.

'You won't find you lose your site, only for an archaeological excavation to suddenly happen,' Dr Pestell pointed out.

'You need to get things recorded - you won't necessarily know if they are a hoard. If they are different types [of coin] then they may not be a hoard – on the other hand, they may be a genuine hoard that's been dispersed by the plough.

'If it's Treasure it automatically belongs to the Crown and is subject to the law of Treasure. If it's declared Treasure by a coroner at an inquest it will be offered to a museum, with finder and landowner eligible for a reward of the full market value of the object.'

If a museum does decide it would like to acquire an item of Treasure, then the National Treasure Valuation Committee will consider the reward payment at one of its meetings at the British Museum (where all prospective Treasure finds are temporarily kept).

Tim, as well as his museums service role, is also one of around nine members of the committee, which meets perhaps six or seven times a year at the London venue.

'We try very hard to work out fair valuations by assessing a provisional value prepared by an independent valuer, working in the antiquities trade. We then examine this and compare it against prices charged by leading dealers, auction houses, and sometimes even eBay to make sure the finder gets the right 'hammer' price. What we're trying to do is establish a fair price for all parties without the dealer's mark-up.'

And they are busy, busy, busy. The most recent meeting, in December, saw Tim and co consider no fewer than 83 cases. Because Tim, wearing his Norfolk Museums and Archaeology Service 'hat', might be interested in acquiring an object discovered locally for the county's collections, he is scrupulous about not taking part in any discussions about such finds. 'I always leave the room if anything from Norfolk is being considered – even if it doesn't involve our museums,' he said.

If a finder or landowner doesn't think the price is fair they can appeal, and suggest reasons why they think the valuation is wrong.

But disputes are very rare, and everyone involved tries their utmost to avoid them. 'I want people to be happy, because those people will come back to us if they find something else,' Tim explained. 'The next fantastic thing they find might not be a Treasure case. So it's essential that we have a good relationship with the finder. If you like, it's 'good business'.'

If an object is declared Treasure and offered to a museum, in most cases they will not take up the offer in any case, and the object will go back to the finder and landowner. This is because a museum might have good examples of a particular object already, or it simply doesn't have the money (and this is where you can help… see later for just how).

'We disclaim [turn down] something like 80 per cent of what we're offered,' Dr Pestell continued. 'We just have to be choosy. We are relying increasingly on the generosity and public-spiritedness of finders. But, really, that's the situation that's existed for 150 years anyway.'

One of the team of experts at the local finds service who helps initially evaluate finds is Dr Adrian Marsden, who has worked there for the past 11 years as a numismatist. 'I deal with coins of all periods, not just the fancy Roman stuff!' he said.

A few weeks ago a detectorist brought in a Roman furniture mount discovered at Woodbastwick – 'Just a bit late for the exhibition, unfortunately' – showing the head of Mercury. 'That was lovely to see, but it's often the material that looks the least remarkable visually that is the more important archaeologically.

'Bricks or tesserae [Roman mosaic fragments] tell you that you have a building there. Or if you have a lot of bronze that could be a Roman counterfeit coin den. It's telling you more than all the fancy stuff,' he explained. 'A gold coin will just tell you that a rich person once walked or rode across a field.'

There are some weak points in the Treasure law. It doesn't always follow that gold and silver items are the most important, or valuable. The most notable example in recent years was the fabulous Crosby Garrett Helmet, dug up in a field in Cumbria in May 2010 by a metal detector enthusiast. Because this stunning Roman object was not made of gold or silver, it was not 'Treasure', so did not automatically belong to the Crown – a situation that would not have been the case in most other European countries which have far stricter antiquities laws.

It was returned to the finder and landowner and was later sold at auction in 2012 for £2.2 million to a private buyer. 'It's been on short-term loan to a few museums already, but it is still not known who now owns it. Hopefully one day it will come into public ownership,' Dr Pestell said. But, for now at least, it's 'the one that got away'.

That was an exceptional case, but the issue of having enough funds to secure the smaller-value but important items for Norfolk is a continuous concern. Long-term loans of objects through patient negotiation and persuasion is one possibility, but fortunately there is also another: the Friends of the Norwich Museums.

The support group has been doing brilliant work since 1921 in help fund acquisitions (not just archaeological ones) for the public collections.

'They are absolutely crucial to what we do,' Dr Pestell said. 'I always encourage people to join the Friends – not only do they get the benefits of membership but they have the satisfaction of knowing they are really making a material difference in helping to preserve Norfolk's heritage.'

See some of the stunning finds made by local detectorists in Roman Empire: Power and People exhibition, which continues at Norwich Castle Museum until April 27.

• www.museums.norfolk.gov.uk • www.friendsofthenorwichmuseums.co.uk