The video below shows how to do the job using Postmix concrete. This is available from your local Builders Merchant.
In the video, a Post Hole Digger is being used. A hole dug by this piece of equipment creates a hole that is about 250mm wide without disturbing the soil around so needs less concrete & when set is much smaller & more secure than a spade-dug hole.
When ordering your poles allow for a minimum of 500mm in the ground (Order your poles here)