Thanks for all your responses so far. To answer a couple of questions, I believe the reason for going deeper was to do with checking the integrity of the existing septic tank. The perc results were poor, and the holes actually gained water during the test. The garden itself is tiered, with the house sitting lower than where the tests were conducted by about 1 ft
The solution proposed was to install a treatment plant, which will pump any liquid after treatment to a soak away on a higher bit of ground. I was happy with this solution, but after the feedback from other drainage companies im not sure whether this solution is based on an inaccurate test in the first place!