In go the back row.

A line was secured from the corner post to a brick. This marked the back row of posts and ran parallel to the boundry line.
We measured out 2m from the corner post back  and tied on a piece of rope which hung down to ground level and indicated exactly where to dig the next post hole.
When the hole was ready we inserted the post and checked It's level relative to the corner post. I had a good margin for error as the plan was to cut the posts of with a grinder after installing the gravel boards.
The first gravel board was slotted in between both posts. It's height not critical at this stage but it had to be made level by use of chocks. Then line up the free post with the line and level the post in horizontal and vertical. Keep checking everything. Small adjustment can be made with a crow bar and the job is much easier when your hole has a nice flat bottom. When happy with everything start packing as before with concrete. Taking care not to disturb the setting we installed one one post in line with the others.