Cement

• General use cement
• Supplied in a waterproof plastic bag

Although this type of cement has many uses, when mixed with 20mm ballast, it is very good for making dry mix concrete suitable for fixing fence posts.
Mix the ballast straight out of the bag with cement in a ratio of 7 ballast to 1 cement. Do not add any extra water. Mix well. Feed the mixture into the hole around the post a few inches at a time and ram it up hard using a spare piece of timber or a Post Hole Tamper, checking as you go that the post is vertical with a level. Fill the hole around the post with the dry mix up to about 75mm (3") below ground level. Fill the top with soil.
Your post should now be firm. It will absorb moisture from the ground and set hard within a few days.