This coin was found very recently by Oliver Jackson. I was asked to provide further details, including a valuation.
The coin is a halfpenny of Henry VI. In the legend on the obverse there is a rosette before REX. On the reverse the legend reads VIL LA CAL IS, with a mascle after LA and a rosette after IS. Therefore, this coin was struck at Calais during the rosette mascle coinage of Henry VI. In the Standard Catalogue it is listed as number 1871.
This is a reasonably decent example of the reign, denomination, type and mint and to a collector it should be worth £40 – £50.