Once home and with the aid of a magnifying glass I could make out the following, large crown, "XVI.s", "VI D" - plus a much smaller indented crown with an "I" below it.
The indented crown with the "I" below identify this coin weight as relating to King James 1st ( reign 1603-1625 ). James was the first Stuart monarch to rule England ( bear in mind he was already King James VI of Scotland ).
With the aid of my Spink coin book I found that during the reign of James 1st one of the gold coins produced was known as the Ryal - this was initially valued as 15 shillings but between the years 1612-1619 the value was increased to be 16 shillings and 6 pence - this matches the "XVI.s" ( 16 shillings ) and "VI D" ( 6 pence ) markings on the weight. So this find is approx 400 years old.
The reverse of the coin weight shows the design of the Ryal coin itself - this feature a spiky sun ( not unlike a "spur" - hence "Spur Ryal" ) within a circle containing eight arches.
My example weighs 5.64g which appears to be less than other known examples ( typically just over 6g ).
Note that From 1632 by Act of Parliament all square coin-weights were made illegal due to the fact that
'many of them, which were in common use were too heavy , and others too light, so that men bought and received by one weight, and sold and delivered by another'.
More info about coin weights of England can be found on this link here
Update - this find has now been recorded onto the Portable Antiquities Scheme ( PAS ) run by the British Museum - see link below
Link to PAS entry for this item