Dear Steve
There is an area perhaps even a road called Stenalees in the Ecclesiastical District of Treverbyn (St PetersChurch) in the civil Parish of St Austell - Treverbyn
The Ward of Treverbyn covers an area to the North of St Austell see this site for a map showing Stenalees :-
http://www.restormel.gov.uk/index.cfm?articleid=4247 Governed by Restormal Borough Council.
so that must be the place on the birth certificate. I found Stenalees just by entering it in the main search engine. It looks as if although born in 1912 your father's birth may not have been registered until 1915 which is very strange and also illegal as parents have 3 months to register a birth. Could the second date be when a copy of the certicate was obtained ? I can't find William and his wife on the 1901 as it may well be that they married after 1901 anyway.
Do you use the FreeBMD website ? That is very helpful but I have looked at it and could not find William's birth on there in 1912 or 1915 but they are adding further records all of the time.
http://www.freebmd.org.ukI think I would be looking very closely at other children born in St Austell at that time to see if they had the same parents as William particularly the Leonard Percival Clift I mentioned. As he is male if he was your father's brother he may still have decendants named Clift. I think the link I found between the two Mills sisters is very interesting - let me know if you ever prove it.
best wishes
Carol