Yes Bluebell the official advice to plant on trenches applies to poor-draining soil.
For you, much better on the flat and with 3in compost on top, since they need to stay moist in this dry spring. Water them in as well.
I echo Tris’ comments on spacing, 3×3′ is good and in year’s one and two you can plant some veg between. The extra space pays off from about year five. Or if they have room to root outwards, that helps.
Happy planting.
