I have dealt with stony clay for a long time, and I have found that plants dislike finding well defined layers underground. Seaweed woud be good, but I would wash it first (to remove salt) chop it up as finely as possible, and cultivate it into the bottom of the trench. It would also be good to make the new soil mix at least 1/4 of the native soil, and cultivate the first cm into the bottom of the trench, to make a transition layer between the old and new soils. Any sudden change in soil texture can actually stop drainage.

griselinia is a lovely hedging plant, but likes free draining moist soil. In addition to the seaweed and advice from tugbrethil, you will need to introduce some grit - john innes no.3 might help?