I don't know about the particular spelling "Forestun", but it's quite possible that this is one of the variant spellings that have occured due to problems with literacy and the perception of the name. Although there is certainly a Forston, England, there are also many variations as the name is passed by word of mouth. I personally have this problem everytime I tell a clerk in a store what my surname is. I've had them spell it Foston, Foreston, Forester, Foson, and a variety of other ways. I've come to assume that historically records also have brought out some of these "creative" spellings. So, don't rule "Forestun" out completely, but don't rely on it either, as a variation of Forston.