Presuming it's real, it's an Alfred the Great silver penny, minted by the moneyer Ceneferth somewhere between 885 - 895 AD. It apparently should weigh about 1.26g, but that might have some variability. I can't find anything about diameter. I'm not sure if you new any of that or not, but sorry if it's redundant. The photo below is presumably a real one. From the 2008 auction site: https://www.cngcoins.com/Coin.aspx?CoinID=116072
Wow, I hope it's real! Alfred the Great is my fifty-something great grandfather. Yes, I'm a Scot, but my grandmother descended from Anglo-Saxons.
My feeling is that this isn't a coin that you own, but one you would like to buy. Viking coins are typically not cheap (they can cost hundreds to thousands of dollars). So if this coin is on an auction site and the price seems a little too good, you may want to be careful.