The annual UK Mineral Statistics volume produced by the British Geological Survey used to include maps showing the areas of existing Crown Estate licenses & leases, and if i remember correctly the last edition I saw (a few years ago) suggested most of the Dolgellau belt was covered by existing licences. Perhaps it is possible to make an arrangement with a licence holder?
There were a number of well known legal cases which excluded the Crown from control of gold and silver when these were worked as a byproduct from working other minerals. Devon & Cornwall in the 19th century produced huge quantities of silver, but as this was contained in galena all rights remained with the mineral rights owner not the Crown.