You will find the Big Wood between the castle, Mansion House and the Avenue Gate. It is shaped like a deformed rectangle. Looking at the image you will see the "Old Road" runs along the side of the wood, shown along the bottom, with the castle to top left of image and mansion house to the top middle. The Avenue gate is located middle right of image.
The Big Wood had many hidden tracks you could walk though which was good as a kid growing up there. There were a badgers set in the centre of the wood and two fox sets near the bottom right. Just to the left centre of the wood was a dump, here all the decomposable garden material was thrown out here and usually set alight.