There are a number of these about - though it is a really super find! I'm not certain if they all meant to circulate as contemporary counterfeits. I like to think they were but some of them might also be gaming tokens or something of the sort.
Here are some ebay listings for the contemporary shilling. There are also sixpences and half crowns. They can come in brass or copper. Some have remains of the silver plating and some seem to have no evidence of it. Search "non regal" or "contemporary" 1819 half crown to see others.
Thanks for your help, its exciting to find anything with age, that was my first find on a little garrett 150, think i'll end up hooked to this metal detecting.