im just building a 6ft fence right round my garden just now.its hard work.need 8 ft posts and dig down 2ft which is a task.get postcrete from bq £3.70 per bag when you buy ten or morethey only come in like 25k bags..you will need 2 bags per post.fill the hole between 1/3 and half with water and pour in the postcrete.simple as that.good luck with it as it aint easy and dont envy people who do this for a living.