There was an interesting little piece on Radio 4's Farming Today about metal detecting (4 mins in).
A few comments on it.
1. Not all items are recorded as the piece says, only items over 300 years old that are voluntarily reported to a Finds Liaison officer (FLO) who work for the Portable Antiquities Scheme (PAS) are recorded. There is no legal requirement to report finds to a FLO. Many detectorists do not report their 300 years old to their local FLO.
There is a legal requirement to report possible Treasure to the local Coroner. Usually Treasure finds are reported the local FLO instead of the Coroner as they act as a go between. The legal requirement to report possible treasure come from the Treasure Act 1996.
2. Any responsible detectorist should have their own specific insurance rather than relying on the farmers public liability insurance. If I was a farmer I would want to see the detectorists insurance.
3. If the farmer is interested in the history of their land they may wish to make sure they ask the detectorist to show them everything that they have found and make sure items are recorded with the FLO.
A farmer might also wish to ask the detectorist to print out and show them some of their personalised PAS finds records (A detectorist can create a username on the PAS website and the finds that they record can get linked to the username. This shows it was the detectorist that reported the find).
4. As to 'lucrative diversification' then a farmer should be aware that it's not just shinny coins that are worth money. Beehive thimbles, buckles, spurs, crotal bells and all sorts of other things can be commonly found by detectorists and can be worth a fair few quid. Some detectorists might think that they can buy you off with a bottle of whiskey in exchange for them not showing you stuff they then keep / sell without you knowing.