Ancient Tree

Browsing the Woodand Trust Ancient Tree Register I noticed this enormous oak on the Penshurst Estate. At 11m girth it's the largest I have seen so far. It was labelled as being on private land but the Penshurst Estate is criss crossed with public footpaths and it did not look like I would have to venture that far from a footpath to find it. 
Also on the Woodland Trust site is a table for estimating the age of an oak by girth which goes up to  9m  =  1000 years old.  I thought only yews lived that long. Another site (Hungerford Museum) says an oak grows at 1.88cm /year which would make the oak 580 years old.  Thats the 1400s, Henry VI on the throne.  I am thinking  the Hungerford age is more likely because Penshurst Place itself was built in 1341.