More progress today after a big change of plan. We discovered an old water tank in the middle, of the plot which is only about 15cm (6 inches) below the surface. We could of built raised beds over it giving a total soil depth of around 38cms (15 inches.) Instead we decided on a major redesign to leave the area clear. There are now five, 2m x 1m beds and one, metre square bed (not yet filled.) This has increased the growing area to 11 square metres using the same number of boards with two 1m lengths left over. They might end up as part of a planter for the centre area.
The beds are now in place and have been filled with a soil and composted manure mix. All that is needed is some green waste compost to top off the beds and we can start planting.