"There is no reliable way of calculating any figures for detectorists who do not record nor indeed the number of recordable finds they make. You are not talking apples and apples."

I agree Co.
So let's take for the sake of argument the lowest number of non-recorders that could be suspected and apply the lowest number of finds it could be expected they find, then halve the consequent estimate.... but after all that, the knowledge is STILL "ours" and they've taken it away! Numbers aren't the answer to questions of rightness and ownership.