JamesP wrote:
I have a mamoth undertaking, I have to plant native hedge and trees
around a four acre boundary as part of my planning approval.

Hedgelaying is a fairly complex skill, if you want the hedge to keep
people or animals out then you don't just stick in trees every few feet
I'm afraid, you actually need to put in a temporary fence, plant
saplings and then 'lay' them after a year to make the hedge into a hedge
rather than a line of trees. Its different if you're using yew or privet
- but I assume you're going for a native deciduous mixed hedge.

Given the length you have to do (probably 700 yards?) You will need some
specialist advice and assistance. This might help.
http://www.hedgelaying.org.uk/