Having contacted the Portable Antiquities Scheme, I have discovered that the coin we found at the weekend is not a coin but rather is a French Jeton, a token that was used in Medieval times as part of complex calculation. Sort of like a bead on an abacus, I think.
But yes, its Medieval! It has lain in the soil for 500 or 600 years since being dropped.
The next stage is to take it along to a surgery that the Portable Antiquities Scheme runs at various locations around Kent to have it more accurately dated and logged.
Its a very special item to have found and only on our second outing with the detector. Beginners luck or what.