Hansard
Your dentist might be the person to help - they have the radiography equipment to detect the stones, and while they probably will not treat the problem themselves, they can arrange referral to the local dental hospitals oral surgery department who should be able to deal with it - possibly under local rather than general anaesthetic.
In any event get it looked at - pain originating in the floor of the mouth needs to be investigated.
Unofficially - If it is a stone (more likley in the duct rather than the gland itself) the old wives tale cure for this was to bite a lemon - this will stimulate massive salivery flow and may eject the stone (bit like bursting a spot) however it can be painfull and is certainly not garanteed to work! - your choice.