In the ground, dig out your native soil to a depth of 1/2 meter, and sift it through a 1/2 cm screen, to remove any rocks or other debris. Test the pH of the soil, and adjust with lime or sulfur, as recommended on the package. Mix the sifted soil with medium-coarse sand, about 4 parts soil to 1-2 parts sand, depending on how sandy the soil is already. If your soil is black and very crumbly, add even more sand, since too much organic matter will cause the roots to twist and branch. The final piece of the puzzle is seaweed--here in the States, we use commercially available kelp meal, but you may have other sources there in the UK. Of the kelp meal, we use about 1/2 liter for every square meter of bed. Mix the first dose with the soil and sand, and apply another dose on the surface in mid season. If your soil is very poor, a bit of blood meal will help at the beginning of the season, but be cautious, since too much will have the same effect as too much organic matter.