Sorry for not having got back sooner. I've now actually seen the item 'in the flesh' and was surprised how small it is - (on my screen the 20p piece is near enough double size) - and also how good is the detailing of the hammer head.
I should have taken some measurements of the object but on the basis that the 20p is a tad over 20mm diameter, the head is well under that, but the pin must be in the order of 40mm long.
It is pretty heavy at that hammer end and would rather discourage the thought of a cuff link.
Yes, it was found in a ploughed field, and hence the mangled 'pin'!!
Anyway thanks for your suggestions - I suspect it might remain a mystery item!!
Rob