Only one is known to exist and most likely, only one was ever minted.
The last British Farthing was minted in 1956.
Such a coin does not exist. The Royal Mint minted no Half-Farthing coins in 1840 and 1841. The Half-Farthing was only minted every few years, and was minted specifically for use in Ceylon (Sri Lanka). The Half-Farthing was not legal tender in Britain until 1842.
There was no 1831 Third-Farthing coins minted. The Third-Farthing coin was produced for use in Malta.
The British copper Halfpenny and the Farthing were both minted in 1697. Both coins are near identical, the Farthing being smaller than the Halfpenny.
The Royal Mint produced no Half-Farthing coins after 1856.
British Farthings were never minted in silver, and no Farthings were minted from 1808 to 1820 inclusive.
Such a coin does not exist. The last British Farthing was minted in 1956 and they were demonetised in 1960.
With the possible exception of traders tokens (with the traders business name on them), there were no "Australian" Farthings ever produced. The only Farthings to ever circulate in Australia were British Farthings.
There were no British Farthings minted from 1776 to 1798 inclusive.
There were no British Farthings minted from 1808 to 1820 inclusive.
No. Coins have been in use as a form of currency for well over 3,000 years. It has recently been found that Farthings were first minted in or about 1216 AD, about 60 years earlier than previously thought. Prior to the Farthing coin being minted, a Farthing was created by cutting a Penny into quarters.
Farthings were not minted from silver, only bronze in 1901. If you have a silver Farthing it would be because somebody has plated it. Modified coins have no collector value.